Add rubygems 3.1.0, remove rubygems 3.0.6
Add rubygems 3.1.0, remove rubygems 3.0.6

for stack(s) cflinuxfs3 [#170304776]

cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack