fix travis failure, move to clang 3.6
fix travis failure, move to clang 3.6

alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,Reactive-Extensions/RxCpp,ReactiveX/RxCpp,alexeymarkov/RxCpp,Reactive-Extensions/RxCpp,ReactiveX/RxCpp,ReactiveX/RxCpp,alexeymarkov/RxCpp