Fix: Agnoster theme added venv name even if disabled

See http://www.virtualenv.org/en/latest/index.html\?highlight\=virtual_env_disable_prompt\#activate-script for info regarding this.

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 ]]; then
+  if [[ -n $virtualenv_path && -z $VIRTUAL_ENV_DISABLE_PROMPT ]]; then
     prompt_segment blue black "(`basename $virtualenv_path`)"
   fi
 }