Define externally accessible contants with 'var' instead of 'const'

Just as we did with classes, define other constants that are (or
may be) used from other modules with 'var' to cut down on warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/misc/weather.js

@@ -11,7 +11,7 @@ const PermissionStore = imports.misc.permissionStore;
 const Util = imports.misc.util;
 
 // Minimum time between updates to show loading indication
-const UPDATE_THRESHOLD = 10 * GLib.TIME_SPAN_MINUTE;
+var UPDATE_THRESHOLD = 10 * GLib.TIME_SPAN_MINUTE;
 
 var WeatherClient = new Lang.Class({
     Name: 'WeatherClient',