Added commands to garbage collect PermGenSpace

While creating a ModPack, I ran out of PermGen space. There used to be an "Increase PermGen Space" checkbox, but is no longer there. I added the JVM arguments to Garbage Collect the PermGen space along with the normal memory. This way, no one should run out of PermGen space because it is garbage collected.

I have not tested this on other operating systems besides Mac. Without these arguments the ModPack did not launch; and with them, it did.