Support RSpec 3 and Mongo 4

This pull request combines:
* https://github.com/evansagge/mongoid-rspec/pull/127: Support for the new RSpec3 expectation protocol by @aomega08
* https://github.com/evansagge/mongoid-rspec/pull/125: Bumps mongoid to 4.0.0.beta1 / Updates documents and indexes specs by @phstc