Add RSpec3 support. Details follow.

* Maintains backward-compability with RSpec 2.14 (previous version don't
have the new expectation syntax so they're not supported).
* Tests green in Rspec 2.14, Rspec 2.99.0.beta1 and 3.0.
* Syntax changes.
* Deprecation fixes.
* Loosen Gemfile dependencies up to RSpec 4.