Config fixes to make the app run in production

I made the following fixes:

1. Updated database.yml. Previously was using sqlite adapter in production. I removed that so it will use postgres (which is what heroku expects).

2. Add pg gem to gemfile so that postgres will work in production.

3. Added 'rails_12factor' gem to enable heroku to serve static assets in production. This is why the CSS wasn't compiling before.