Do not manually update system RubyGems (#6355)

Travis CI ships compatible system RubyGems now:
https://github.com/travis-ci/travis-ci/issues/8969#issuecomment-360288970
> I have repackaged the 2.5.0 archive for Linux to include RubyGems 2.7.4,
> which should have the fix for this issue. Please restart the affected
> jobs, and let us know how they go for you.