Rubocop updates to app/volt/ 

I took a stab at fixing some of the Rubocop offenses.  These changes are localized to `app/volt/`...

I was going to try for the whole project today but Rubocop was like: 
![flipping out](http://i.imgur.com/FhO62Ii.gif)

I'll try to do a directory at a time going forward.

This PR brings in the following changes:

- Bumped Ruby version to 2.1.5.  Rubocop supports this and all opal/rspec tests pass on 2.1.5. (I hope this is ok! :grin:)
- Added a rake task for easily running rubocop. `rake rubocop`
- Reduced the number of offenses from something like 30+ to 10. The 10 remaining I left because i'm not yet comfortable enough with Volt to take on major modifications.