[infer][PR] Add missing dependency in Travis

Summary:
Adding dependencies via `apt` can fix the broken builds on master.

Should close https://github.com/facebook/infer/issues/1174, and close https://github.com/facebook/infer/issues/1125
Pull Request resolved: https://github.com/facebook/infer/pull/1179

Differential Revision: D18328499

Pulled By: artempyanykh

fbshipit-source-id: 64807214a

.travis.yml

@@ -7,6 +7,8 @@ cache:
     - ${HOME}/.opam
     - facebook-clang-plugins
 before_install:
+  - sudo apt-get update
+  - sudo apt-get install -y libmpfr-dev
   - wget -O ${HOME}/opam https://github.com/ocaml/opam/releases/download/2.0.3/opam-2.0.3-x86_64-linux
   - chmod +x ${HOME}/opam
   - export PATH=${HOME}:${PATH}