Make offline embedded <div> plots responsive.

This PR enables responsiveness in offline plots, where `output_type = 'div'` and not just if  `output_type = 'file'`. I've been making use of [Django-JET](https://github.com/geex-arts/django-jet) dashboard widget plugins in order to embed a Plotly graph. 

Before this change I would have to reload the page every time I resized the browser window in order to make the plot resize to the new sizes.

Author: Ismael-VC

Diff for: plotly/offline/offline.py

@@ -511,7 +511,8 @@ def plot(figure_or_data, show_link=True, link_text='Export to plot.ly',
                 get_plotlyjs(),
                 '</script>',
                 plot_html,
-                '</div>'
+                resize_script,
+                '</div>',
             ])
         else:
             return plot_html