Fix failing builds on Travis CI by using latest rubygems
Fix failing builds on Travis CI by using latest rubygems

Currently, builds are failing on Travis CI. e.g.) https://travis-ci.org/bbatsov/rubocop/jobs/194136460

The cause is from rubygems, and it has been fixed.

See also. https://github.com/sickill/rainbow/issues/48

Note
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Builds seem to be failing for different reason in JRuby.
https://travis-ci.org/bbatsov/rubocop/jobs/194136459

This change hasn't fixed the problem.

jonas054/rubocop,jonas054/rubocop