Don't assume default DerivedData location

[enriquez]

UIAuto magically finds your app bundle if it is in ~/Library/Developer/Xcode/DerivedData/. This location may be different if it has been customized in Xcode... so we should first parse ~/Library/Preferences/com.apple.dt.Xcode.plist to find out the actual location of DerivedData.

Related code: https://github.com/enriquez/uiauto/blob/master/lib/uiauto/instruments.rb#L86