Add rubygems 2.7.4, remove 2.6.13 [#154440553]
Add rubygems 2.7.4, remove 2.6.13 [#154440553]

Signed-off-by: Sam Coward <f2d9ddb83d985bcf331a113cd487aeceb5e618e4@pivotal.io>

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