Fix Ruby 2.5.0 build failure - RubyGems 2.7.4 is required.
Fix Ruby 2.5.0 build failure - RubyGems 2.7.4 is required.

https://travis-ci.org/tzinfo/tzinfo/jobs/322314021
https://github.com/travis-ci/travis-ci/issues/8978

tzinfo/tzinfo