Make sales_price compatible with spree 2.1 and Rails 4.x

I've made the gem compatible with Rails 4 and Spree 2.2

Branch 2-1-stable may not be the best to pull-request against, but that is the branch that I took as base. 
This changes two things in one line: 
Spree 2.2 no longer uses "permalink" attribute but instead uses "slug".
Rails 4 no longer has the "magic find_by_foo" finders, but instead urges you to use `find_by(attribute: "value")`