Conversion to Yasnippet

Here's a somewhat opinionated port with some additions and the following changes:

* There seems to be no reason to use `Pathname` when including spec_helper.
* [thoughtbot style guide](https://github.com/thoughtbot/guides/tree/master/style#testing) proscribes starting `it` block descriptions with `should`, and I agree that it looks repetitive.
* Unlike #34, I'm claiming `exp` for `expect(...).to`, which was [introduced in RSpec 2.11](http://myronmars.to/n/dev-blog/2012/06/rspecs-new-expectation-syntax) and is planned to be made the only default syntax in a future version. The block form is used less often, so the trigger can be longer.