Ruby code style guide implementation

- https://github.com/bbatsov/ruby-style-guide
- https://github.com/bbatsov/rubocop
What I am saying is that, this is not changed while https://github.com/apiaryio/redsnow/pull/42/files#diff-10f89a34ff7ff6c3f42db75936ffa3acL219 is changed but both of them are almost the same things.