[Fix] gatling-maven: child JVM does not exit if parent JVM exits

This is a fix for issue Issue #1047. The fix is simply set a ShutdownHookProcessDestroyer to the DefaultExecutor. 

To fix this issue, I followed the last bullet point in this [DefaultExecutor's Doc](http://commons.apache.org/proper/commons-exec//apidocs/org/apache/commons/exec/DefaultExecutor.html)

I have tested this on my Windows machine.