Allow client side javascript to access rubygems API show action

When creating a new gem, I find myself wanting to know if there's already a gem with that name. I wrote a really basic page that uses client side JS to make a call to the rubygems api. It's really small and fast since it's pretty much just that javascript.

The only issue is that rubygems.org doesn't allow this kind of access. This pull request sets headers to allow client side javascript to access the rubygems show action.

You can check out the page I made at http://johncolvin.github.io/isthereagemnamed/. (I'm going to make it look much nicer)

Let me know what you think and thanks for all your work on rubygems!