allow chaining after $.fn.datepicker

Change-Id: I555c958de874252d62e7f67898e59107fb589bad
Reviewed-on: https://gerrit.instructure.com/5408
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>

public/javascripts/instructure_helper.js

@@ -2519,6 +2519,7 @@ I18n.scoped('instructure', function(I18n) {
     }
     this.realDatepicker(options);
     $(document).data('last_datepicker', this);
+    return this;
   };
   $.fn.date_field = function(options) {
     options = $.extend({}, options);