Update PaymentMethod.available for display_on

This is required because of the following commit to spree:

https://github.com/spree/spree/commit/ce8f4121b08fc7420867d5909cca327d04bde12d

This added an available method which collides with the store restriction method.  This will make multi-domain play nicer with spree master.

This should be merged in to to master as well.