Ruby code style guide implementation

- https://github.com/bbatsov/ruby-style-guide
- https://github.com/bbatsov/rubocop
Okay, this inconsistent style are the only thing left to be fixed. Compare https://github.com/apiaryio/redsnow/blob/abtris/rubocop/lib/redsnow/sourcemap.rb#L42-L47 and https://github.com/apiaryio/redsnow/blob/abtris/rubocop/lib/redsnow/sourcemap.rb#L62-L67.