Update readme to point to develop branch travis

Author: hardbyte

README.rst

@@ -11,9 +11,9 @@ python-can
    :target: https://python-can.readthedocs.io/en/stable/
    :alt: Documentation Status
 
-.. |build| image:: https://travis-ci.org/hardbyte/python-can.svg?branch=master
+.. |build| image:: https://travis-ci.org/hardbyte/python-can.svg?branch=develop
    :target: https://travis-ci.org/hardbyte/python-can
-   :alt: CI Server
+   :alt: CI Server for develop branch
 
 
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