Quiet warnings for utf8 options

Tmux 2.2 removed `utf8` and `status-utf8` options. Since we still want
to keep these options for earlier tmux versions we're redirecting the
output to /dev/null when setting these commands. This way tmux 2.2 (and
above) users won't get any errors when running this plugin.

Author: Bruno Sutic

sensible.tmux

@@ -67,11 +67,11 @@ key_binding_not_changed() {
 main() {
 	# OPTIONS
 
-	# enable utf8
-	tmux set-option -g utf8 on
+	# enable utf8 (option removed in tmux 2.2)
+	tmux set-option -g utf8 on 2>/dev/null
 
-	# enable utf8 in tmux status-left and status-right
-	tmux set-option -g status-utf8 on
+	# enable utf8 in tmux status-left and status-right (option removed in tmux 2.2)
+	tmux set-option -g status-utf8 on 2>/dev/null
 
 	# address vim mode switching delay (http://superuser.com/a/252717/65504)
 	if server_option_value_not_changed "escape-time" "500"; then