Update subscription_preference_spec.rb (#342)

- remove check to see if validation message is still the same. since it is enough to say that validation failed without some attr. and because we did not customize the error message, and are mainly relying on validation message coming from rails (which has been updated from Rails 4 to Rails 5)

spec/models/subscription_preference_spec.rb

@@ -5,7 +5,7 @@
     it 'should require a user' do
       without_user = build :subscription_preference, user: nil
       expect(without_user).to_not be_valid
-      expect(without_user).to fail_validation user: "can't be blank"
+      expect(without_user).to fail_validation
     end
 
     it 'should require a category' do