[SPARK-12107][EC2] Update spark-ec2 versions

I haven't created a JIRA. If we absolutely need one I'll do it, but I'm fine with not getting mentioned in the release notes if that's the only purpose it'll serve.

cc marmbrus - We should include this in 1.6-RC2 if there is one. I can open a second PR against branch-1.6 if necessary.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #10109 from nchammas/spark-ec2-versions.