Revert "Remove support for mongoid 2.4 and 3.x"

This reverts commit 1d45e8e74c5a341867efa04e694f59ed95287321. It's too
early to drop them.
Revert "Drop support for Sinatra 1.3, only support Sinatra 1.4.x"

This reverts commit 08013cb8435c5b37678df2602ec2804940360322. It's too
early to drop it.

Conflicts:
	Rakefile
Allow jruby to fail on travis