use cmake3 instead of cmake (#3857)
use cmake3 instead of cmake (#3857)

When trying to build this dockerfile i was getting the below error:

`CMake 3.3 or higher is required. You are running version 2.8.12.2`

Using cmake3 resolves this.
alfonsokim/nupic,numenta/nupic,alfonsokim/nupic,rcrowder/nupic,numenta/nupic,alfonsokim/nupic,numenta/nupic,rcrowder/nupic,rcrowder/nupic