Remove pypi downloads badge

The download stats are no longer available, according to the badge.

README.md

@@ -1,7 +1,6 @@
 # ipywidgets: Interactive HTML Widgets
 
 [![Version](https://img.shields.io/pypi/v/ipywidgets.svg)](https://pypi.python.org/pypi/ipywidgets)
-[![Downloads](https://img.shields.io/pypi/dm/ipywidgets.svg)](https://pypi.python.org/pypi/ipywidgets)
 [![Build Status](https://travis-ci.org/jupyter-widgets/ipywidgets.svg?branch=master)](https://travis-ci.org/jupyter-widgets/ipywidgets)
 [![Documentation Status](http://readthedocs.org/projects/ipywidgets/badge/?version=latest)](https://ipywidgets.readthedocs.io/en/latest/?badge=latest)
 [![Join the chat at https://gitter.im/ipython/ipywidgets](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/jupyter-widgets/Lobby)