Add Travis build badge to README.md

Link back to https://travis-ci.org/jelmer/dulwich

README.md

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 This is the Dulwich project.
 
+[![Build Status](https://travis-ci.org/jelmer/dulwich.png?branch=master)](https://travis-ci.org/jelmer/dulwich)
+
 It aims to provide an interface to git repos (both local and remote) that
 doesn't call out to git directly but instead uses pure Python.