fix deprecation failure_message

lib/cancan/matchers.rb

@@ -4,11 +4,11 @@
     ability.can?(*args)
   end
 
-  failure_message_for_should do |ability|
+  failure_message do |ability|
     "expected to be able to #{args.map(&:inspect).join(" ")}"
   end
 
-  failure_message_for_should_not do |ability|
+  failure_message_when_negated do |ability|
     "expected not to be able to #{args.map(&:inspect).join(" ")}"
   end
 end