Send 'QUIT' instead of `kill -9` or 'TERM' to halt instruments processes

When testing against iOS 8 devices, sending a `kill -9` or `TERM` signal to instruments will result in the ScriptAgent process running on the device to become orphaned and un-responsive.

```
MobileGestaltHelper[909] <Error>: libMobileGestalt MobileGestalt.c:273: server_access_check denied access to question UniqueDeviceID for pid 796 
ScriptAgent[796] <Error>: libMobileGestalt MobileGestaltSupport.m:170: pid 796 (ScriptAgent) does not have sandbox access for re6Zb+zwFKJNlkQTUeT+/w and IS NOT appropriately entitled
ScriptAgent[703] <Error>: libMobileGestalt MobileGestalt.c:534: no access to UniqueDeviceID (see <rdar://problem/11744455>)
```

The first calabash Scenario will run, but run-loop will be unable to relaunch the app for the next Scenario.

This is closely related to `calabash_exit`.  For example, if test suite calls `calabash_exit`, this bug may not occur because the parent instruments process *might* die.


* Fixes "Isolating the ScriptAgent no access to UniqueDeviceID bug" https://github.com/calabash/run_loop/issues/34
* Partial fix for "Errno::EINTR: Interrupted system call @ rb_sysopen"  https://github.com/calabash/run_loop/issues/35