Switch have support to use the extracted gem

[JonRowe]

We extracted the have(n) etc matchers to a seperate gem, use that in rspec-rails 3.x

[JonRowe]

I've realised this clashes with @soulcutter #810

Which path do we prefer gang?

/cc @myronmarston @alindeman @samphippen

We need to merge one of them...

[myronmarston]

I prefer to keep all the logic in the rspec-collection_matchers gem. If we keep this logic in rspec-rails, I think it sends a mixed message about what the preferred way of asserting on collection size is.