Only include ActiveRecord matchers if AR is defined

Fixes cases where you may be using ActiveModel without ActiveRecord.

Also add a test for loading without ActiveRecord.

The Cucumber feature creates a Gemfile containing ActiveModel and
shoulda-matchers, and then asserts that both can be loaded without any
errors.

The ActiveModel version used will match that of the Rails version
specified in the Appraisal Gemfile; and if no such version can be
determined, the latest will be used.

Closes #480.