lib/theme: use _command_exists() in rbenv_version_prompt()

themes/base.theme.bash

@@ -413,7 +413,7 @@ function rvm_version_prompt() {
 }
 
 function rbenv_version_prompt() {
-	if which rbenv &> /dev/null; then
+	if _command_exists rbenv; then
 		rbenv=$(rbenv version-name) || return
 		rbenv commands | grep -q gemset && gemset=$(rbenv gemset active 2> /dev/null) && rbenv="$rbenv@${gemset%% *}"
 		if [[ "$rbenv" != "system" ]]; then