Fix for issue #578: Rubocop logo

Based on the suggestions in issue #578, I propose this.

Illustrator and photoshop files available if needed. Can also do other formats if required (GIF, JPEG).

T-shirts and stickers available at Redbubble:

[Rubocop](http://www.redbubble.com/people/suranyami/works/11118765-rubocop)

Feedback is welcome.