Different syntax for setting zsh precmd

Old versions of bash choke on the += syntax, even though it's in a chunk
of code for another shell. Closes #122 - h/t @atannen.

z.sh

@@ -219,7 +219,7 @@ if compctl >/dev/null 2>&1; then
             }
         fi
         [[ -n "${precmd_functions[(r)_z_precmd]}" ]] || {
-            precmd_functions+=(_z_precmd)
+            precmd_functions[$(($#precmd_functions+1))]=_z_precmd
         }
     }
     _z_zsh_tab_completion() {