showing the mail-in-a-box version might fail if git isn't actually installed

The user might acquire the sources via some means other than a git clone. On Vagrant, the files come in via Vagrant. So test for git before running `git describe`.

setup/start.sh

@@ -53,7 +53,9 @@ fi
 if [ "$PRIVATE_IPV6" != "$PUBLIC_IPV6" ]; then
 	echo "Private IPv6 Address: $PRIVATE_IPV6"
 fi
-echo "Mail-in-a-Box Version: " $(git describe)
+if [ -f /usr/bin/git ]; then
+	echo "Mail-in-a-Box Version: " $(git describe)
+fi
 echo
 
 # Run some network checks to make sure setup on this machine makes sense.