Implement autocorrection for Rails/ReadWriteAttribute cop.

This implements autocorrection for the Rails/ReadWriteAttribute cop.

It is my first contribution to rubocop, therefore I'm not sure it fits your coding conventions;-)
Instances with explicit receivers are not marked as offenses, and therefore also not autocorrected.

I could change it to also record them as offenses, and implement the according autocorrection. Otherwise I'd add some spec documenting the current behaviour.

What do you think?