Reduce the number of versions we keep retro compatibility

[rodrigopinto]

Mongoid is currently at the version 7 but we supporting the all the gem since the version 2.

It is a good thing that mongoid-rspec can offer a set that covers last 5 versions, in counterpart, we are starting to write conditional code to keep this compatibility and it is starting to complicate the code in a way that does not make sense anymore, at least in my point of view.

With this in mind, I thought to reduce the past mongoid versions that this repo at the HEAD supports, and keep this backward compatibility only via tags as we already have.