Revert "Force activemodel dependecy to 4.0.x"

This reverts commit 157d2df3d3aef6f06a3320ae070a20866564bb72.
This reverts commit 5235c02dc7c53c706078a199c370d8f22b156dc3.

[fixes #3680]

Loose ActiveModel dependency again, so we can release Mongoid 4.0, with
AM 4.0 and 4.1 support.
Fix 3 out of 4 failing callbacks tests on ActiveModel 4.1

Tests for running callbacks were fixed.
The last failing one is: spec/mongoid/interceptable_spec.rb#431

Conflicts:
	mongoid.gemspec
Define callback so chain is not emtpy
Dont allow failures on AM 4.1