CMake: make sure python library is in 2.x series

When looking up the python libraries, if you don't specify a minimum
version, it can pull in the python 3 libraries if not installed. Since
things don't build with python 3, make sure to restrict to the 2.x series.
Merge pull request #252 from simonsonc/python-ver

CMake: make sure python library is in 2.x series