DRY tasks/bower.rake

There is no more `dsl` namespace for rake tasks. Tasks are the same as for `bower.json` also for `Bowerfile` configuration files.

PS: Ross, cant find your email to ping you so I ask it here: could you add me as a collab for this repo? That would be great! Gonna add some tests, travis-ci and other cool stuff. Also I'd like to have your mail to contact you if you don't mind. You can find my one @ my profile.