Shutdown iOS Simulator before killing simulator jobs to avoid bad simulator state.

In Xcode 6 simulator could be in different states: creating, shutdown, booting, booted or shutting down. If simulator is killed and didn't have chance to reset its state to `shutdown` and stuck in state `booted` then xctool will not be able to use that simulator anymore until the state is reset to `shutdown`. 

Luckily CoreSimulator framework provides methods to reset iOS Simulator state to `shutdown` which xctool will be using every time before killing all simulator jobs.