Declare a Ruby version in the Gemfile. Let's go for 2.1.0!
Fix asset pipeline compilation on Heroku.
Update a few gems.

Also remove the omniauth gem from the Gemfile. That's simply a
dependency that should be resolved by Bundler. This way, after doing
`bundle update`, the omniauth-oauth2 gem was updated which was needed,
since the previous version contained a vulnerability.
Tweak some details.

* Use more fine-grained GitHub OAuth scopes
* Change Facebook popup dimensions
* Declare multiple JavaScript variables with a single `var`.
Improve link_to_login_with helper method.

It now behaves a little more like the regular link_to method.