tmux: don't use -bg since 2.9

Signed-off-by: Alejandro Mery <amery@geeks.cl>

files/.tmux.conf

@@ -7,6 +7,8 @@ run-shell "\
 
 # https://github.com/tmux/tmux/blob/master/CHANGES
 #
+# 2.9
+# * individual -fg/-bg removed, use -style instead
 # 2.4
 # * vi-copy syntax changes
 # 2.2
@@ -96,7 +98,7 @@ bind k send-keys -R \; clear-history
 
 # http://stahlke.org/dan/tmux-nested/
 set -g status-bg colour40
-setw -g window-status-current-bg colour40
+if-shell -b 'test $TMUX_VERSION -lt 209' 'setw -g window-status-current-bg colour40'
 
 bind -n C-t new-window -a
 bind -n S-left  prev