plugins/rbenv: use `_command_exists`

clean_files.txt

@@ -90,6 +90,7 @@ plugins/available/history.plugin.bash
 plugins/available/hub.plugin.bash
 plugins/available/jump.plugin.bash
 plugins/available/node.plugin.bash
+plugins/available/rbenv.plugin.bash
 plugins/available/textmate.plugin.bash
 plugins/available/xterm.plugin.bash
 

plugins/available/rbenv.plugin.bash

@@ -1,7 +1,10 @@
+# shellcheck shell=bash
 cite about-plugin
 about-plugin 'load rbenv, if you are using it'
 
 export RBENV_ROOT="$HOME/.rbenv"
 pathmunge "$RBENV_ROOT/bin"
 
-[[ `which rbenv 2>/dev/null` ]] && eval "$(rbenv init - bash)"
+if _command_exists rbenv; then
+	eval "$(rbenv init - bash)"
+fi