update travis to use Xcode 6.4
update travis to use Xcode 6.4

alecthomas/AwesomeCache,remzr7/AwesomeCache,remzr7/AwesomeCache,namanhams/AwesomeCache,aschuch/AwesomeCache,icepy/AwesomeCache,nickoneill/AwesomeCache,alecthomas/AwesomeCache,namanhams/AwesomeCache,RyanTech/AwesomeCache,icepy/AwesomeCache,alecthomas/AwesomeCache,takeshineshiro/AwesomeCache,RyanTech/AwesomeCache,takeshineshiro/AwesomeCache,remzr7/AwesomeCache,nickoneill/AwesomeCache,icepy/AwesomeCache,aschuch/AwesomeCache,takeshineshiro/AwesomeCache,nickoneill/AwesomeCache