agnoster theme not showing virtualenv status

agnoster theme was checking if the default virtualenv prompt was turned off and not showing its own... so there was no way to see which virtualenv was selected.

themes/agnoster.zsh-theme

@@ -137,7 +137,7 @@ prompt_dir() {
 # Virtualenv: current working virtualenv
 prompt_virtualenv() {
   local virtualenv_path="$VIRTUAL_ENV"
-  if [[ -n $virtualenv_path && -z $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
+  if [[ -n $virtualenv_path && -n $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
     prompt_segment blue black "(`basename $virtualenv_path`)"
   fi
 }