Added support for modern matcher syntax

* `failure_message` renamed to `failure_message_for_should`
* `negative_failure_message` renamed to `failure_message_for_should_not`

Adds support for shoulda-matchers >= 1.5, but also keeps support for
shoulda-matchers < 1.5.
Merge pull request #22 from thoughtbot/modern-matcher-syntax

shoulda-context is incompatible with shoulda-matchers >= 1.5