Update to latest RubyGems
Update to latest RubyGems

- see https://github.com/bundler/bundler/issues/5370

Signed-off-by: Sam Smith <03a07629efb0a02c7bbe7cc340a4243da1ab861a@gmail.com>

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci