Work around rubygems/bundler mismatch error.

Author: myronmarston

travis/.travis.yml

@@ -5,6 +5,7 @@ cache:
   directories:
     - ../bundle
 before_install:
+  - gem update --system # https://github.com/travis-ci/travis-ci/issues/8978#issuecomment-354036443
   - unset _JAVA_OPTIONS
   - "script/clone_all_rspec_repos"
   # Note this doesn't work on JRUBY 2.0.0 mode so we don't do it, the excluded versions are broken on Ruby 2.3