Update README.rst (#184)

Change PNG Badges to SVG

Author: tcyrus

README.rst

@@ -6,7 +6,7 @@ MessagePack for Python
 :version: 0.4.6
 :date: 2015-03-13
 
-.. image:: https://secure.travis-ci.org/msgpack/msgpack-python.png
+.. image:: https://secure.travis-ci.org/msgpack/msgpack-python.svg
    :target: https://travis-ci.org/#!/msgpack/msgpack-python