.pbxproj converted to plist old format

when running liftoff on a new project, it changes the format of pbxproj. 

see diff after liftoff process: https://gist.github.com/netbe/7779087

This is related to the version of xcodeproj. So I updated it and  did not quite solve the problem directly (see https://github.com/CocoaPods/Xcodeproj/issues/84).

It finally did the trick when i got `xcproj`  installed :
```
brew install xcproj
```

kind of weird of `xcproj` needs to be installed, maybe the guys @cocoapods know more how to solve this.