Set VIRTUAL_ENV_DISABLE_PROMPT to 1

Otherwise powerline prompt is being prefixed by `(virtualenv-name)` in zsh which 
looks rather ugly. In bash prompt is always overridden by PROMPT_COMMAND, thus 
there is no actual need to set this variable.

powerline/bindings/bash/powerline.sh

@@ -43,6 +43,7 @@ _powerline_prompt() {
 }
 
 _powerline_setup_prompt() {
+	VIRTUAL_ENV_DISABLE_PROMPT=1
 	if test -z "${POWERLINE_COMMAND}" ; then
 		if which powerline-client &>/dev/null ; then
 			export POWERLINE_COMMAND=powerline-client

powerline/bindings/zsh/powerline.zsh

@@ -111,6 +111,7 @@ _powerline_setup_prompt() {
 		fi
 	done
 	precmd_functions+=( _powerline_set_jobnum )
+	VIRTUAL_ENV_DISABLE_PROMPT=1
 	if test -z "${POWERLINE_NO_ZSH_ZPYTHON}" && { zmodload libzpython || zmodload zsh/zpython } &>/dev/null ; then
 		precmd_functions+=( _powerline_update_counter )
 		zpython 'from powerline.bindings.zsh import setup as _powerline_setup'