[infer][PR] Fix opam remove error

Summary:
Please see [CONTRIBUTING.md](./CONTRIBUTING.md) for how to set up your development environment and run tests.

https://github.com/facebook/infer/pull/1181

https://github.com/facebook/infer/issues/1240
Pull Request resolved: https://github.com/facebook/infer/pull/1241

Reviewed By: dulmarod

Differential Revision: D20766779

Pulled By: jvillard

fbshipit-source-id: 8ed01b230

.travis.yml

@@ -33,7 +33,7 @@ before_install:
   - export OPAMVERBOSE=1
 install:
   # ensure infer isn't installed in this switch, then deal with dependencies
-  - opam remove infer
+  - opam remove infer || true
   - opam update --upgrade
   - opam pin add --no-action infer .
   - if [ "$TRAVIS_OS_NAME" != "osx" ]; then opam depext --update infer; fi
@@ -41,7 +41,7 @@ install:
 script:
   - opam install infer
 before_cache:
-  - opam remove infer
+  - opam remove infer || true
 os:
 - linux
 - osx