Merge pull request #598 from shivaram/master.

Update spark_ec2 to use 0.9.0 by default

Backports change from branch-0.9

Author: Shivaram Venkataraman <shivaram@eecs.berkeley.edu>

Closes #598 and squashes the following commits:

f6d3ed0 [Shivaram Venkataraman] Update spark_ec2 to use 0.9.0 by default Backports change from branch-0.9