Setup rubocop.

Closes #702.
That's probably a good idea, but it hasn't been a problem in the other repos:

* https://github.com/rspec/rspec-expectations/blob/318cda1ba796ee0882e9665ff6a7b116e118b0a7/Rakefile#L74-L78
* https://github.com/rspec/rspec-support/blob/master/Rakefile#L26-L30

...and it hasn't been a problem yet.  Still, it's a good idea.