Test mysql and postgres on travis ci instead of sqlite

The main changes are splitting the `database.yml.example` file into `database.yml.mysql`, `database.yml.postgresql` and `database.yml.sqlite`, and adjusting the `rake fulcrum:setup` task to copy  `database.yml.#{ENV['DB']}` to `database.yml` (defaults to sqlite).


Also, the changes in the gemfile seem a bit ugly to me, but I couldn't think of a better way to do it. Keen for suggestions.