Change the optimization arguments to use G1GC

[mstojcevich]

Enables G1 garbage collector with string deduplication. This garbage collector is parallel, concurrent, and provides shorter and more consistent GC pause times. Reduces duration of GC pauses that hang the game, especially with large heap sizes. Haven't tested with Java 7, if it breaks, then remove the string deduplication argument.

[progwml6]

i'm going to set g1gc on java 8 users by default and ignore this current list. it was designed before mods were using java 7 as a minimum see #988