Update sandcastle.js

Windows doesn't support signals ('SIGHUP' causes error ENOSYS in Windows). As such, the child processes keep running. Now sending 'SIGTERM' under windows. From NodeJS documentation:

"Note that Windows does not support sending Signals, but node offers some emulation with process.kill(), and child_process.kill(): - Sending signal 0 can be used to search for the existence of a process - Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional exit of the target process."