Fixing GitHub user login/creation

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 `1.1.2` 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-github` gem instead of `1.1.2`.

Running `bundle install`, `bundle update` after including `omniauth-oauth2` naturally allowed `omniauth-github` to bump itself to its latest/working version.  Users were successfully created after that.

Facebook also seems to work without pinning the version either, so I removed the version pin.

* I'm on Rails 4, Ruby 2