While we wait for 1.6.11...

I've taken a number of the pull requests that address rails 4 issues, but also just general query bugs and rolled them up into single PR... the idea being if people want to use a more up to date version of CanCan.

In a nutshell:

* It moves up to 1.9.3, (1.8.7 support could be added back in if that's important to people)
* Fixes several bugs (shallow routing, nested conditions, class level scoping, multiple rule issues with associations)
* Fixes several Rails 4 incompatibilities (scoping, deprecation warnings)
* Adds strong_parameters support
* Uses `.ruby-version` instead of `.rvmrc`

This PR combines:

https://github.com/ryanb/cancan/pull/726
https://github.com/ryanb/cancan/pull/864
https://github.com/ryanb/cancan/pull/871
https://github.com/ryanb/cancan/pull/906
https://github.com/ryanb/cancan/pull/935
https://github.com/ryanb/cancan/pull/966
https://github.com/ryanb/cancan/pull/971
https://github.com/ryanb/cancan/pull/974
https://github.com/ryanb/cancan/pull/977
https://github.com/ryanb/cancan/pull/988

Thank you to all the other contributors!