update wakatime badge with branch

Author: alanhamlett

README.rst

@@ -18,7 +18,7 @@
     :target: https://pypi.python.org/pypi/wakatime
     :alt: Supported Python Versions
 
-.. image:: https://wakatime.com/badge/github/wakatime/wakatime.svg
+.. image:: https://wakatime.com/badge/github/wakatime/wakatime.svg?branch=master
     :target: https://wakatime.com/
     :alt: Code time tracker