[BUGFIX beta] Use the jQuery polyfill for Node.prototype.contains

Currently Ember is broken in FF17 (I'm not sure if others) because both `Node.contains` and `Node. compareDocumentPosition` are missing.

This swaps out the polyfill for the jQuery version.

Technically it's a BUGFIX release, but because of FF being evergreen and no other open bug reports, it's probably safe to wait.

ping @rwjblue