Add directed graphs for Bio-Formats latest jobs

This PR proposes to use the `graphviz` sphinx extensions as a way to document the relationship between CI jobs in the Contributing documentation. This PR applies it to the latest Bio-Formats jobs before a potential extension to the OMERO jobs.

Note this PR constitutes a breaking change as it would enforce `graphviz` to be installed on the developers machines as well as the docs jobs in order to compile the documentation (see Travis build change).

/cc @melissalinkert @hflynn