progressbar: set_value() should update()

set_value() didn't call update() and thus had no visible effect on the
progressbar.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/widget/progressbar.lua.in

@@ -116,6 +116,7 @@ end
 function set_value(pbar, value)
     local value = value or 0
     data[pbar].value = math.min(1, math.max(0, value))
+    update(pbar)
     return pbar
 end