ensure_inclusion_of instead validate_inclusion_of and alike?

[pablox-cl]

I was seeing that some of the rspec-mongoid matchers are similar to the ones from shoulda matchers, but some have minor differences, like using ensure instead of validate. Shouldn't they be similar to be more consistent?

[rodrigopinto]

@pablox-cl The mongoid-rspec has no expectation to keep the api consistent with shoulda-matcher, however if you submit a PR that implements an alias, maybe we can merge it.