@dylanfareed => Prepare for public release.

Various bits and pieces for a public release, see the commit descriptions.

I changed ENV['gravity_url'] to prefer ENV['ARTSY_API_URL'], keeping the old one for backwards compat. Ideally this shouldn't even be there and the strategy should be entirely configurable without any ENV.

Please wait for the [build](https://travis-ci.org/artsy/omniauth-artsy/builds/33522948) to succeed.
I'll make sure we transition away from using ```gravity_url``` in other internal client apps.