[DISCUSS] Miniumum hadoop version on master to 2.7

[Siddharth Seth]

The 2.7 release has been available for almost 2 years now (
http://hadoop.apache.org/releases.html). Should we change the minimum
version of Hadoop to 2.7.0.
The main motivation at the moment would be to avoid writing a few shims
(well, 1 for now). e.g. YARN-3427.