In FreeBSD bash is located at /usr/local/bin/bash instead of /bin/bash

While attempting to build consul on FreeBSD 10, the build script won't run without alteration.  I changed the #!/bin/bash line to use /usr/bin/env to get the location of bash from the environment.  This should allow it to build on Linux/Mac OS X/FreeBSD.

Tested on:
Fedora 20
Ubuntu 14.04
Mac OS X 10.9
FreeBSD 10