There was a mistake in sbt build file ( introduced by 012bd5fb ) in which we set the default to 2048 and the immediately reset it to 1024.

Without this, building Spark can run out of permgen space on my machine.

Author: Reynold Xin <rxin@apache.org>

Closes #103 from rxin/sbt and squashes the following commits:

8829c34 [Reynold Xin] Allow sbt to use more than 1G of heap.