Fix build

Added comment

Author: Michael Wright

.travis.yml

@@ -16,6 +16,8 @@ env:
 
 before_install:
  - |
+    # work-around for issue https://github.com/travis-ci/travis-ci/issues/6307
+    # might not be necessary in the future
     if [ "$TRAVIS_OS_NAME" == "osx" ]; then
      command curl -sSL https://rvm.io/mpapis.asc | gpg --import -
      rvm get stable