Switch badges from 'pypip.in' to 'shields.io'

The badge hosting at 'pypip.in' has been broken for a while
now, it was supposed to be fixed in the following PR(s):

- badges/pypipins#39
- badges/pypipins#38
- badges/pypipins#37

That does not appear to have actually happened and/or is not
getting addressed, so switch to a more reliable badge hosting
site that provides the same badge information.

Change-Id: Ia1fd048e8f1810c4aaabc24bd01c7f0c18b79fd6

README.rst

@@ -1,11 +1,11 @@
 Oslo Messaging Library
 ======================
 
-.. image:: https://pypip.in/version/oslo.messaging/badge.svg
+.. image:: https://img.shields.io/pypi/v/oslo.messaging.svg
     :target: https://pypi.python.org/pypi/oslo.messaging/
     :alt: Latest Version
 
-.. image:: https://pypip.in/download/oslo.messaging/badge.svg?period=month
+.. image:: https://img.shields.io/pypi/dm/oslo.messaging.svg
     :target: https://pypi.python.org/pypi/oslo.messaging/
     :alt: Downloads