If you need to use a raster PNG badge, change the '.svg' to '.png' in the link
Remove zope.interface==3.6.1 from .travis.requirements.txt.
The Twisted version is dynamic and we should allow it to pick its
required Zope version. See
https://travis-ci.org/isislovecruft/bridgedb/jobs/51769343#L1229 for an
example of how this fails in Travis CI builds.
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