Make compatible with Spree 2.2
Bump to spree_core 2.3.0.beta
use try to avoid NoMethodError for nil class
fix referred users association by adding a source
Merge branch 'bug/fix_referred_users_association' into 2-2-stable

Conflicts:
	app/models/spree/user_decorator.rb
	spree_affiliate.gemspec
Merge branch 'bug/fix-nil-error' into 2-2-stable