Update to RSpec 3 syntax

The red Travis badge was making me sad. :cry:

Seriously, though… while all of the specs were passing in RSpec 2.x, the Gemfile did not specify any version, so anyone diving straight in with `bundle install` and trying to run the specs with 3.x would see a lot of failures.

I made minimal changes for all specs to pass in RSpec 3.x without any errors or deprecation warnings, and then I added `~> 3.0` to the Gemfile.
I think we can just use expect - I don't think we use should anywhere...