places-menu: Fix JS warning

Define class PlacesManager with 'var' instead of 'const', to shut
up a gjs warning when accessing it from the extension.

https://bugzilla.gnome.org/show_bug.cgi?id=787291

extensions/places-menu/placeDisplay.js

@@ -220,7 +220,7 @@ const DEFAULT_DIRECTORIES = [
     GLib.UserDirectory.DIRECTORY_VIDEOS,
 ];
 
-const PlacesManager = new Lang.Class({
+var PlacesManager = new Lang.Class({
     Name: 'PlacesManager',
 
     _init: function() {