Allow sbt to use more than 1G of heap.

There was a mistake in sbt build file ( introduced by 012bd5f ) 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.

sbt/sbt-launch-lib.bash

@@ -105,7 +105,7 @@ get_mem_opts () {
   local mem=${1:-2048}
   local perm=$(( $mem / 4 ))
   (( $perm > 256 )) || perm=256
-  (( $perm < 1024 )) || perm=1024
+  (( $perm < 4096 )) || perm=4096
   local codecache=$(( $perm / 2 ))
 
   echo "-Xms${mem}m -Xmx${mem}m -XX:MaxPermSize=${perm}m -XX:ReservedCodeCacheSize=${codecache}m"