Update #increment in Mongoid to allow 3.x and 4.x

The syntax of increment method changed from 3.x to 4.x. This commit makes sure
it will work in both of them
Test different Mongoid versions

We support Mongoid 2, 3 and we're aiming to support Mongoid 4.
Changes in gemfiles allow us to check all of them (but not all Rails-Mongoid
versions combinations!)
Fix gemfiles to test properly agains Mongoid 4 and Rails 4 and 4.1
add gemfile for Mongoid 3
Merge pull request #575 from arnvald/mongoid_4

Mongoid 4