The new default Heroku-16 stack is based on Ubuntu 16 which is not yet
supported by the upstream version of the graphviz buildpack.
Switching to a fork until upstream is updated.
Graphviz is installed via heroku-buildpack-graphviz-src and the location
of dot(1) is automatically added to PATH.
There is also heroku-buildpack-graphviz-deb which should be easier to
install, but the Debian package installed has a font style problem.
