Decrease maven & tests heap size.

This should make the travis build pass more consistently.

It seems odd to set such a large heap for maven.  The total available memory is 3GB, so the heap + perm shouldn't exceed that limit.  I've set `Xmx` to 1500m for maven and the tests, which (I hope) will stop the random OOMs.  This actually over-subscribes the memory somewhat, but we should be able to get away with it.