Allow sbt to use more than 1G of heap.

There was a mistake in sbt build file ( introduced by 012bd5fbc97dc40bb61e0e2b9cc97ed0083f37f6 ) 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.
[SPARK-1194] Fix the same-RDD rule for cache replacement

SPARK-1194: https://spark-project.atlassian.net/browse/SPARK-1194

In the current implementation, when selecting candidate blocks to be swapped out, once we find a block from the same RDD that the block to be stored belongs to, cache eviction fails  and aborts.

In this PR, we keep selecting blocks *not* from the RDD that the block to be stored belongs to until either enough free space can be ensured (cache eviction succeeds) or all such blocks are checked (cache eviction fails).

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #96 from liancheng/fix-spark-1194 and squashes the following commits:

2524ab9 [Cheng Lian] Added regression test case for SPARK-1194
6e40c22 [Cheng Lian] Remove redundant comments
40cdcb2 [Cheng Lian] Bug fix, and addressed PR comments from @mridulm
62c92ac [Cheng Lian] Fixed SPARK-1194 https://spark-project.atlassian.net/browse/SPARK-1194