Add http:// prefix to redirect_uri param for GitHub auth, closes #1237

This closes #1237

The GitHub OAuth API docs specify that a `redirect_uri` param include the scheme prefix (eg, `http://` ). Check out: http://developer.github.com/v3/oauth/#redirect-urls for details and examples.

You can actually see this fix working in production by taking the following steps:

1. Log out of Exercism
2. Go to a submission-specific URL, for example http://exercism.io/submissions/14e9cec1407cac4e34e2129f 
3. Right-click on the "Log in via GitHub" button, and copy the link
4. Paste the URL into your address bar, and where it says `...&redirect_uri=exercism.io...`, add "http://", so that it says `...&redirect_uri=http://exercism.io...`
5. Click enter to follow that link, and see the glory of the functioning redirect_uri param!

Let me know if you think this requires test coverage in and of itself.