Use shields.io instead of pypip.in.

It seems that the pypip.in has been unreachable. https://github.com/badges/pypipins/issues/39
diff --git a/README.rst b/README.rst
index 4f5c646..98e8d17 100644
--- a/README.rst
+++ b/README.rst
@@ -1,7 +1,7 @@
 Cryptography
 ============
 
-.. image:: https://pypip.in/version/cryptography/badge.svg?style=flat
+.. image:: https://img.shields.io/pypi/v/cryptography.svg
     :target: https://pypi.python.org/pypi/cryptography/
     :alt: Latest Version