point `source` to `https://rubygems.org`

Since the recent rubygems.org exploit the rubygems team have been recommending that everyone update their Gemfiles to have `source` point to `https://rubygems.org`

From the Rubygems chatroom on Freenode

> over http, a man in the middle could provide you a malicious gem

I'm closing [Pull Request 721](https://github.com/emberjs/data/pull/721) (because I was having trouble squashing/rebasing the commits) and issuing a new pull request here.