Use batman-rails instead of vendored batman

This is an effort to replace the vendored version of batmanjs with the batman-rails gem.

The current code seems to work, but it has some issues:

- `batman-rails`'s assets paths are not registered in the configuration of the end project (the project using dashing-rails). I don't know what I would have to do for it to work. Am I missing something here?
- I can't confirm if every widget is still working after the upgrade from 0.14.1 to 0.16.1. Are there tests?