Lower server memory to 1.5GiB

The server was killed today by Java failing to allocate enough memory. 
Apparently we are reserving too much, so let's drop that down to 1.5GiB.

This comes with a drop in maximum player count to 15, until we can be sure
that it still works for over that number.

@nfreader, can you give me a thumbs up to merge this in since the
player cap has dropped?