Improve and document TravisCI configuration

* Add Travis to README.
* Color the RSpec output.
* Create Postgres database.
* Set up xvfb for Capybara Webkit.
* Turn off email notifications.
* Remove unnecessary `ruby-` prefix to Ruby version.
* Use consistent formatting.
Add `bin/setup` for contributors

* Generate binstubs in `bin/setup`.
* Use `mkdir .git/safe` convention in `bin/setup`.
  https://github.com/thoughtbot/suspenders/commit/2ec7108
* Add missing `rake` binstub to version control.
* Update and improve `CONTRIBUTING.md`.
Don't generate binstubs, move rake back to CONTRIBUTING.md