On my OS X El Capitan machine `which` returns 1 if it can't find the binary. This kills the bootstrap script at the crucial moment when it detects that something needs to be installed. This change ignores unsuccessful exit codes from `which`. The man page for my `which` speaks of a `-s` switch that could be used instead of the `if [ -z`, but I don't know how portable this is.

This fixes Issue 850.