Remove `schema_for` in favor of `$ref`

https://github.com/seanpdoyle/json-matchers/issues/4
Address `failure_message_when_negated` missing

In order to integrate with `RSpec`, we need to declare the failure message
methods. Instead of declaring those on the matcher itself, we declare them in
the `rspec` integration file.

This makes future integration with MiniTest easier and cleaner.