require marked.min and call it via marked.marked.

In v6 jupyter notebook components/marked/lib/marked no longer exists. The extension silently falls over. 
Hack around puts local copy of marked.min from cdnjs.com into the python-markdown directory.
See Issue ipython-contrib#1638

src/jupyter_contrib_nbextensions/nbextensions/python-markdown/main.js

@@ -13,7 +13,7 @@ define([
     'require',
     'notebook/js/cell',
     'base/js/security',
-    'components/marked/lib/marked',
+    'nbextensions/python-markdown/marked.min',
     'base/js/events',
     'notebook/js/textcell'
 ], function(IPython, $, requirejs, cell, security, marked, events, textcell) {
@@ -60,9 +60,9 @@ define([
                         var html;
                         if (out_data.msg_type === "error") {
                             var text = "**" + out_data.content.ename + "**: " +  out_data.content.evalue;
-                            html = marked(text);
+                            html = marked.marked(text);
                         } else if (out_data.msg_type === "stream") {
-                            html = marked(out_data.content.text);
+                            html = marked.marked(out_data.content.text);
                             var t = html.match(/^\s*<p>([\s\S]*?)<\/p>\s*$/); //strip <p> and </p> that marked (maybe) adds and we don't want
                             html = t !== null ? t[1] : html;
                             var q = html.match(/^&#39;([\s\S]*?)&#39;$/); // strip quotes from strings
@@ -84,11 +84,11 @@ define([
                                     var png = ul['image/png'];
                                     html = '<img src="data:image/png;base64,' + png + '"/>';
                                 } else if (ul['text/markdown'] != undefined) {
-                                    html = marked(ul['text/markdown']);
+                                    html = marked.marked(ul['text/markdown']);
                                 } else if (ul['text/html'] != undefined) {
                                     html = ul['text/html'];
                                 } else {
-                                    html = marked(ul['text/plain']);
+                                    html = marked.marked(ul['text/plain']);
                                     // [\s\S] is used to also catch newlines
                                     var t = html.match(/^\s*<p>([\s\S]*?)<\/p>\s*$/); //strip <p> and </p> that marked adds and we don't want
                                     html = t !== null ? t[1] : html;