Do not run specs against AR 3.2 and older and Ruby head

I don't think they'll be supported.
AR edge is stable! :sparkles:
Fix a bug where the friendly url for the first page is broken

fixes #554
Use minitest on rbx against specs for mongoid 2.4
Do not allow the specs for mongo_mapper/mongoid 2.4/rbx to fail on travis
Revert "AR edge is stable! :sparkles:"

This reverts commit 5f7c5a27c9d3ea20f865cd2ded9111c9e5096144.

Conflicts:

	.travis.yml
oops :fire: