Enable Graphviz on Heroku.

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.
Merge pull request #431 from knu/heroku-graphviz

Enable Graphviz on Heroku.