fix UIAlertView test behavior

you can't just do a removeFromSuperview, it adds a whole `UIWindow` hierarchy and breaks future functional tests.

spec/motion/core/app_spec.rb

@@ -25,7 +25,7 @@
 
   describe '.alert' do
     after do
-      @alert.removeFromSuperview
+      @alert.dismissWithClickedButtonIndex(@alert.cancelButtonIndex, animated: false)
     end
 
     describe "with only one string argument" do