Fixing TravisCI after ruby upgrade (see travis-ci/travis-ci#8978)

Author: kpumuk

.travis.yml

@@ -21,6 +21,10 @@ notifications:
   recipients:
     - [email protected]
 
+before_install:
+  - gem update --system
+  - gem install bundler -v 1.16.1
+
 before_script:
   - curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter
   - chmod +x ./cc-test-reporter