Remove $NTA reference

Supports https://github.com/numenta/nupic/issues/809.

This PR should be merged together with https://github.com/numenta/nupic/pull/917 (pip-installable). This is because `lib_cppregion.so` will be installed too. In the current implementation, this file is obtained from $NTA/lib.
What happens if the command failed to execute, like there's no `python` or no `nupic` to import? Should a more clear error info to be thrown?