BUG: Really use the basic ipynb export template

The previous code intended to set the template to basic, but it doesn't
really work and the full template was being used.

nikola/plugins/compile/ipynb.py

@@ -82,9 +82,9 @@ def compile_html_string(self, source, is_two_file=True):
         """Export notebooks as HTML strings."""
         if flag is None:
             req_missing(['ipython[notebook]>=2.0.0'], 'build this site (compile ipynb)')
-        HTMLExporter.default_template = 'basic'
         c = Config(self.site.config['IPYNB_CONFIG'])
         exportHtml = HTMLExporter(config=c)
+        exportHtml.template_file = 'basic'
         with io.open(source, "r", encoding="utf8") as in_file:
             nb_json = nbformat.read(in_file, current_nbformat)
         (body, resources) = exportHtml.from_notebook_node(nb_json)