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.

Nice. This obviously could never have worked before without a preexisting ./env. It looks like 849af8935d764510561165f165bfc44837c0f0b0 is what broke it. Maybe you could take a look at what that commit removed and consider bringing it back? It used to check ownership of python in case sudo is required for pip and easy_install. I'd think that would still be necessary if it ever was necessary.