Updating so that GitHub login returns email in the callback

The GitHub API was not returning the email address upon login:  `email=nil`.  As a result, no user was created and exceptions occurred.

Upgrading `omniauth-github` to its latest version fixed it, but the latest version depended on the `omniauth-oauth2` gem.  I've added `omniauth-oauth2` here. Without `omniauth-oauth2`, we were stuck at `1.0.2` of the `omniauth-oauth2` gem instead of `1.1.2`.

Running `bundle install`, `bundle update` will naturally bump `omniauth-github` to a working version.  Users were successfully created after that.

Facebook also seems to work without pinning the version either, so I removed the version pin.