Use RuboCop for ruby/rails linting?

I was playing around with RuboCop over the weekend and added it to Bridge Troll to test it out. Not sure if this'll be interesting to anyone or not, but I wanted to kick off a discussion since I'm already sitting on 80% of the code. @tjgrathwell if it's going to make you crazy to get build failures from lint errors, feel free to just close.

**Pro:**
- Occasional contributors who are new to ruby or rails get lots of fun style & linty feedback.
- Core contributors don't have to use up bandwidth giving stylistic feedback.
- Occasionally catches confusing weirdness like unused variables.
- Project code looks more consistent.

**Con:**
- Core contributors get irritated by sometimes having to switch `%w()` to `%w{}` (Just an example — we can configure the rules to be whatever).
- One more working part to deal with; new contributors might find it intimidating.

To make the diff more manageable, see it [without whitespace](https://github.com/eanakashima/bridge_troll/compare/railsbridge:master...eanakashima:rubocop?w=1).