Update RubyGems for Ruby 2.5 on Travis
Update RubyGems for Ruby 2.5 on Travis

The combination of Bundler 1.16.1 and RubyGems 2.7.3 causes a known
issue.

https://github.com/travis-ci/travis-ci/issues/8978#issuecomment-354036443

newcontext-oss/kitchen-terraform,newcontext-oss/kitchen-terraform,newcontext-oss/kitchen-terraform,newcontext-oss/kitchen-terraform,newcontext/kitchen-terraform