Which git fix

This is my first "hello, world" contrib for salt.
I have some other fixes batched up but I will start with the simplest pull request I have
and progress from here.  Thanks ...

On stock centOS 6.3 machines without "git" installed the command "which git"
returns an error to stderr and as a consequence every Salt command prints
this same error to the screen. This one-line fix to "salt/version.py" prevents this
from occurring.