add missing opam step in macos source install

Summary:
@public
The "eval `opam config env`" step was missing from the macos instructions.

See https://github.com/facebook/infer/issues/26

Test Plan: No functional change

INSTALL.md

@@ -48,6 +48,7 @@ Install the OCaml dependencies:
 
 ```bash
 opam init --comp=4.01.0  # (answer 'y' to the question)
+eval `opam config env`
 opam install sawja.1.5 atdgen.1.5.0 javalib.2.3 extlib.1.5.4
 ```