fix a regression in 'rake simulator' introduced by 01315ce30 - also, the 'deployment_target' env variable becomes 'target'

lib/motion/project.rb

@@ -61,7 +61,7 @@ end
 desc "Run the simulator"
 task :simulator => ['build:simulator'] do
   app = App.config.app_bundle('iPhoneSimulator')
-  target = ENV['deployment_target'] || App.config.deployment_target
+  target = ENV['target'] || App.config.sdk_version
 
   # Cleanup the simulator application sandbox, to avoid having old resource files there.
   if ENV['clean']