move the default build to 2.7 hadoop

[animeshtrivedi]

None of us are using 2.6 hadoop. All Spark versions are also for 2.7 build. if by mistake i do mvn build i get 2.6 packages that later on creates problems with spark.

I propose to make the default build to 2.7, with the backward option for 2.6. Next time anyone doing a pull request please consider setting this (or I will do, what ever happens first)