Fixed auto update.

Paramater substitution instead of command subsitution was being used
causing the calculation of time since last check to be incorrect.

tools/check_for_upgrade.sh

@@ -16,7 +16,7 @@ then
     _update_zsh_update && return 0;
   fi
 
-  epoch_diff=$((${_current_epoch} - $LAST_EPOCH))
+  epoch_diff=$(($(_current_epoch) - $LAST_EPOCH))
   if [ $epoch_diff -gt 6 ]
   then
     echo "[Oh My Zsh] Would you like to check for updates?"