[tmux] tmux set -g mouse-mode on doesn't work

So this option has been renamed in version 2.1 (18 October 2015)

From the changelog:

 Mouse-mode has been rewritten.  There's now no longer options for:
    - mouse-resize-pane
    - mouse-select-pane
    - mouse-select-window
    - mode-mouse

  Instead there is just one option:  'mouse' which turns on mouse support

So this is what I'm using now in my .tmux.conf file

set -g mouse on