Uses the env python, rather than /usr/local

dokku (github.com/progrium/dokku) uses these buildpacks to stub out a private Heroku.  The environments it creates don't have python installed at /usr/local/bin/.

This commit should make the buildpack work for me (as a test, anyway).
Merge pull request #1 from frio/patch-1

Uses the env python, rather than /usr/local