Update to work with spree 1-3-stable

Have updated the readme about the working of this plugin.
Updated gemspec for latest bundle dependencies.
Updated cart override deface which was having issues with ruby 1.9.x and above
Updated QuantityLimit's attribute to accessible.

All in all everything is working with latest Spree 1.3.x, rails 3.1.x or above and ruby 1.9.x or above.

Also, did't see 1-3-stable branch or your repository so had to send PR on master.