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):

- https://github.com/badges/pypipins/issues/39
- https://github.com/badges/pypipins/issues/38
- https://github.com/badges/pypipins/issues/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: I8c7d12c20ee9120f618f64b0a0354628c4f31fc5

README.rst

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