Bump down jdk version to 1.6

Sponge is considering using flow-math to provide a default math implementation in SpongeAPI but we cannot use past jdk6. I've compiled flow-math and it works fine on 1.6 and there is simply no reason for flow-math to be 1.7

I could have made this change myself but I don't want to burn any bridges :p (at least I think I could).

If refused, we can fork this repo but I'd rather not have to maintain a fork.