more precise gcc and Python requirements in INSTALL.md

Summary:
@public
gcc >= 4.7.2 is fine, as is Python >= 2.7. Also reformatted to
80-columns to make it easier to read in a terminal.

Test Plan: None

INSTALL.md

@@ -1,8 +1,10 @@
 #Install Infer
 
-We provide pre-built Infer binaries for Linux and MacOS.
-If you just wish to use Infer, and are not interested in making contributions to it, then these binaries are all you need.
-Otherwise, if you wish to compile Infer, here are also instructions to do so, depending on your operating system.
+We provide pre-built Infer binaries for Linux and MacOS.  If you just
+wish to use Infer, and are not interested in making contributions to
+it, then these binaries are all you need.  Otherwise, if you wish to
+compile Infer, here are also instructions to do so, depending on your
+operating system.
 
 - [Install the Infer binaries](INSTALL.md#install-the-infer-binaries)
 	- [Mac OS X](INSTALL.md#mac-os-x)
@@ -16,11 +18,13 @@ Otherwise, if you wish to compile Infer, here are also instructions to do so, de
 
 ###Requirements
 
-- Python 2.7
+- Python >= 2.7
 
 ###Mac OS X 
 
-Get the latest `infer-osx-vXX.tar.xz` from [infer releases](https://github.com/facebook/infer/releases) and run the commands below in your terminal to install Infer.
+Get the latest `infer-osx-vXX.tar.xz` from [infer
+releases](https://github.com/facebook/infer/releases) and run the
+commands below in your terminal to install Infer.
 
  ```bash
 tar xf infer-osx-vXX.tar.xz
@@ -32,7 +36,9 @@ echo "export PATH=$PATH:`pwd`/infer-osx/infer/infer/bin" \
 
 ###Linux (64 bit)
 
-Get the latest `infer-linux64-vXX.tar.xz` from [infer releases](https://github.com/facebook/infer/releases) and run the commands below in your terminal to install Infer.
+Get the latest `infer-linux64-vXX.tar.xz` from [infer
+releases](https://github.com/facebook/infer/releases) and run the
+commands below in your terminal to install Infer.
 
  ```bash
 tar xf infer-linux64-vXX.tar.xz 
@@ -45,48 +51,60 @@ echo "export PATH=$PATH:`pwd`/infer-0.1-x64-linux/infer/infer/bin" \
 
 ##Install Infer from source
 
-The following instructions describe how to compile Infer on different platforms. 
+The following instructions describe how to compile Infer on different
+platforms.
 
 ###Requirements
 
-- Python 2.7
+- Python >= 2.7
 
 ### Download the Infer repository
 
+```bash
 git clone https://github.com/facebook/infer.git
+```
 
-To analyse C and ObjC, Infer requires clang and the [facebook-clang-plugin](https://github.com/facebook/facebook-clang-plugins). If you wish to analyse only Java/Android code, then you could skip these dependencies. Details below.
+To analyse C and ObjC, Infer requires clang and the
+[facebook-clang-plugin](https://github.com/facebook/facebook-clang-plugins). If
+you wish to analyse only Java/Android code, then you could skip these
+dependencies. Details below.
 
 ###MacOS X 
 
 ####Requirements
 
-- `opam` (Instructions [here](https://opam.ocaml.org/doc/Install.html#OSX))
+- [opam](https://opam.ocaml.org/doc/Install.html#OSX)
 
 ##### Requirements for Java analysis
-- `Java <= 1.7`
-- Android dev setup for analysis of Android apps.
+
+- Java <= 1.7
+- Android dev environment setup for analysis of Android apps.
 
 ##### Requirements for C/ObjC analysis 
-- `XCode <= 6.3, >= 6.1`
-- `clang` (XCode command line tools. You can install them with the command `xcode-select --install`)
+
+- XCode <= 6.3, >= 6.1
+- clang (in XCode command line tools. You can install them with the command `xcode-select --install`)
 
 ###Installation instructions
 
-Install OCaml dependencies:
+Install the OCaml dependencies:
+
 ```bash
 opam init --comp=4.01.0  # (answer 'y' to the question)
 opam install sawja.1.5 atdgen.1.5.0 javalib.2.3 extlib.1.5.4
 ```
 
-If you do not require support for the C/Objective C analysis in Infer, and only wish to analyse Java files, continue with these instructions. By the way, Java 1.8 is not supported.
+If you do not require support for the C/Objective-C analysis in Infer,
+and only wish to analyse Java files, continue with these
+instructions. By the way, Java 1.8 is not supported.
 
 ```bash
 cd infer
 make -C infer java
 export PATH=`pwd`/infer/bin:$PATH
 ```
-To compile support for both Java and C/Objective C, do this instead.
+
+To compile support for both Java and C/Objective-C, do this instead.
 
 ```bash
 cd infer
@@ -97,9 +115,11 @@ export PATH=`pwd`/infer/bin:$PATH
 
 ###Linux
 
-These instructions were tested on Linux (64 Bit), on the following distributions: Debian 7, Ubuntu 14.04 and Ubuntu 12.04.4 LTS.
+These instructions were tested on Linux 64 bits on the following
+distributions: Debian 7, Ubuntu 14.04 and Ubuntu 12.04.4 LTS.
+
+Install the OCaml dependencies:
 
-Install OCaml dependencies:
 ```bash
 sudo apt-get update
 sudo apt-get upgrade
@@ -110,7 +130,10 @@ chmod +x opam
 eval `./opam config env`
 ./opam install sawja.1.5 atdgen.1.5.0 javalib.2.3 extlib.1.5.4 #(then say 'y' to the question)
 ```
-​If you do not require support for the C/Objective C analysis in Infer, and only wish to analyse Java files, continue with these instructions. By the way, Java 1.8 is not supported.
+
+If you do not require support for the C/Objective-C analysis in Infer,
+and only wish to analyse Java files, continue with these
+instructions. By the way, Java 1.8 is not supported.
 
 ```bash
 cd infer
@@ -118,7 +141,12 @@ make -C infer java
 export PATH=`pwd`/infer/bin:$PATH
 ```
 
-To compile support for both Java and C/Objective C, do this instead. If your distribution is Ubuntu 12.04.4 LTS, you need to install `gcc 4.8` and  `g++ 4.8` as well. Follow the following instructions to do that. You may skip this step in other distributions.
+To compile support for both Java and C/Objective-C, do this
+instead. This assumes that gcc >= 4.7.2 is already installed on your
+system. If your distribution is Ubuntu 12.04.4 LTS, you can install
+gcc-4.8 and g++-4.8. Follow the following instructions to do that. You
+may skip this step in other distributions with a recent enough version
+of gcc, for instance Debian 7.
 
 ```bash
 sudo apt-get install python-software-properties
@@ -137,4 +165,4 @@ cd infer
 ./compile-fcp.sh
 make -C infer
 export PATH=`pwd`/infer/bin:$PATH
-```
+```