Ensure that PY_VERSION is set in the user's environment

This resolves the numpy version mismatch some users were seeing.

all credit to @oxtopus
Combine PY_VER and PY_VERSION
env.sh now sets $PY_VERSION so use that here.

@oxtopus
Merge pull request #262 from iandanforth/patch-4

Ensure that PY_VERSION is set in the user's environment
Merge pull request #263 from iandanforth/patch-5

env.sh now sets $PY_VERSION so use that here.