update 1.3.3 release date and add note about upgrading from 1.3.x
update 1.3.3 release date and add note about upgrading from 1.3.x

jashkenas/backbone,ppker/backbone,craigmichaelmartin/backbone,platinumazure/backbone,jashkenas/backbone,craigmichaelmartin/backbone,moimikey/backbone,ppker/backbone,akre54/backbone,platinumazure/backbone,moimikey/backbone,akre54/backbone