Bump samples to java 7

[cam72cam]

In my (limited) testing it looks like java7 should work just fine, even when java 8 breaks forge.

[quat1024]

Sry this is an old comment...! been meaning to pick this project up again

For the actual jdk, I've been using java 8 because it's the easiest to provision with stuff like the gradle toolchains feature, i don't think any older versions are easily available

For sourceCompatibility, im pretty sure minecraft & forge used java 6 at the time so i've been keeping it low to be safe. If 7 works, modders are free to raise it to 7, but i know 8 definitely doesn't work