[SPARK-5122] Remove Shark from spark-ec2

I moved the Spark-Shark version map [to the wiki](https://cwiki.apache.org/confluence/display/SPARK/Spark-Shark+version+mapping).

This PR has a [matching PR in mesos/spark-ec2](https://github.com/mesos/spark-ec2/pull/89).

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

Closes #3939 from nchammas/remove-shark and squashes the following commits:

66e0841 [Nicholas Chammas] fix style
ceeab85 [Nicholas Chammas] show default Spark GitHub repo
7270126 [Nicholas Chammas] validate Spark hashes
db4935d [Nicholas Chammas] validate spark version upfront
fc0d5b9 [Nicholas Chammas] remove Shark