use alternate method to collapse pager from notebook.js as suggested by @Carreau

IPython/frontend/html/notebook/static/js/notebook.js

@@ -130,7 +130,7 b' var IPython = (function (IPython) {'
             } else if (event.which === key.ESC) {
                 // Intercept escape at highest level to avoid closing
                 // websocket connection with firefox
-                that.element.trigger('collapse_pager');
+                IPython.pager.collapse();
                 event.preventDefault();
             } else if (event.which === key.SHIFT) {
                 // ignore shift keydown