setup.py: Add missing dependency for building documentation

In later versions of sphinx the rtd theme needs installing explicitly
as it is no longer included in the main package.

setup.py

@@ -98,7 +98,7 @@ params = dict(
         'test': ['nose', 'mock'],
         'mongodb': ['pymongo'],
         'notify': ['notify2'],
-        'doc': ['sphinx'],
+        'doc': ['sphinx', 'sphinx_rtd_theme'],
         'postgres': ['psycopg2-binary']
     },
     # https://pypi.python.org/pypi?%3Aaction=list_classifiers