Fix logo URL in README
-# <a href="https://github.com/jupyterhub/repo2docker"><img src="https://raw.githubusercontent.com/jupyterhub/repo2docker/3fa7444fca6ae2b51e590cbc9d83baf92738ca2a/docs/source/_static/images/repo2docker.png" height="48px" /> repo2docker</a>
+# <a href="https://github.com/jupyterhub/repo2docker"><img src="https://raw.githubusercontent.com/jupyterhub/repo2docker/8731ecf0967cc5fde028c456f2b92be651ebbc18/docs/source/_static/images/repo2docker.png" height="48px" /> repo2docker</a>
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