Display required Ruby version on rubygem show page

Issue #617 mentioned it would be helpful to display the required ruby version on rubygems.org if one exists.

This PR will display a required version of Ruby for all new gem versions that get added hereafter. However, pre existing rubygems will not automagically show their required ruby version unless we do some sort of data migration. I'm not sure how you would like me to progress on that part, so if you have any thoughts, please let me know.