Update matchers to be compatible with RSpec 3

Thanks for this gem!

This updates the matchers to work with RSpec 3 and not issue deprecations.

I updated the gemspec to rely on RSpec 3 or higher, though with some more work I suppose the changes could be adapted to support both RSpec 2 and 3, but I'm not sure if there's value in that.

The test suite passes as well as the test suite for my application using these matchers.