travis build fails for python 3.6 complete on plotting

[kzecchini]

Not sure what causes this failure, but there is a pydot failure during tests sometimes. The first time I opened the PR there was a failure. When I reran the build on travis, it was successful. Unfortunately the logs are not viewable for old builds that are rerun. This is something to keep in mind and log the full stacktrace/travis output if it happens again.