Get specs running under Rails 4.1.0.beta1

1. Switched to MiniTest for Rails 4.1 compatibility.
2. Add explicit unscoped when checking results in test, so we don't get clobbered by the existing default_scope on the lookup.

The default_scope behavior appears to have changed in Rails 4.1.  Previously the addition of a 'where(active: false)' would override the existing 'where(active: true)' from the default_scope.  That no longer occurs in Rails 4.1.  Instead one must explicitly unscope the default_scope to get the same behavior as in earlier Rails version.

The change I made to address this issue only affects the lookup of matching records in the test, and not the code in the library itself.  I believe it preserves the original intent of the specs.