Use `is_expected.to` instead of `should`

Even though we've switched to the `expect` syntax, we've still been using
`should` for one-liners with an implicit subject. RSpec 3 introduced
`is_expected.to` as an alternative. Myron Marston explains:

> Some users have expressed confusion about how this should relates to the
> expect syntax and if you can continue using it. It will continue to be
> available in RSpec 3 (again, regardless of your syntax configuration), but
> we’ve also added an alternate API that is a bit more consistent with the
> expect syntax

From: [New API for
one-liners](http://myronmars.to/n/dev-blog/2014/05/notable-changes-in-rspec-3#new_api_for_oneliners_)