Move xmodule_runtime.xmodule_instance registration earlier

This allows XModules (specifically CombinedOpenEnded) to use ajax_url
during their init functions (which would, before, have thrown an
exception).

[LMS-1493]

common/lib/xmodule/xmodule/annotatable_module.py

@@ -50,7 +50,7 @@ class AnnotatableModule(AnnotatableFields, XModule):
     icon_class = 'annotatable'
 
     def __init__(self, *args, **kwargs):
-        XModule.__init__(self, *args, **kwargs)
+        super(AnnotatableModule, self).__init__(*args, **kwargs)
 
         xmltree = etree.fromstring(self.data)