Bump to rspec 3, use rspec-mocks instead of rr

Following up to internal discussion, I personally went with rr for more flexibility in times I couldn't do things like `any_instance_of` with rspec, but a lot has changed since then.

Rspec 3 has been in beta for 6 months so it's probably good enough to upgrade. Overall I really like the changes/direction it's going, although slightly annoyed with the changes in expectations.

[Nice summary of the changes here](http://myronmars.to/n/dev-blog/2014/05/notable-changes-in-rspec-3)