Fix #861: configure.sh fails on Mac OS X

https://github.com/OpenBazaar/OpenBazaar/issues/861 

This updated configure.sh works on macs, both on fresh installs and on upgrades from the last release.

Yea I had at one point prepended /path/to/repos/env/bin/ to PATH so it would check ./env/bin for pip and others first, but I've changed it back to the explicit reference now to ./env/bin