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.

Note that changing this to `pip` here will cause the packages to be installed to system instead of ./env, which is not what we want.

Unless I've overlooked something, this needs to remain `./env/bin/pip`