Fix setup.py readme extension

Since I moved README.md to README.rst I have to change it's reference in
the setup.py file. Failure to do so would have resulted in a build
error.

setup.py

@@ -5,7 +5,7 @@
 here = path.abspath(path.dirname(__file__))
 
 # Get the long description from the README file
-with open(path.join(here, 'README.md'), encoding='utf-8') as f:
+with open(path.join(here, 'README.rst'), encoding='utf-8') as f:
     long_description = f.read()
 
 setup(