clear xcode_version value when change xcode dir path

http://hipbyte.myjetbrains.com/youtrack/issue/RM-566

If change the Xcode dir path via "app.xcode_dir", already cache the @xcode_version value with '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild'.
We cannot retrieve iOS 8 simulator device name from Xcode 5.
So, 'iPhone 4s / iOS 7' simulator is always launched as default.

lib/motion/project/xcode_config.rb

@@ -51,6 +51,7 @@ module Motion; module Project;
     end
 
     def xcode_dir
+      @xcode_version = nil
       @xcode_dir ||= begin
         xcode_dot_app_path = '/Applications/Xcode.app/Contents/Developer'