Update Travis config to work with Python 3.7
Update Travis config to work with Python 3.7

https://github.com/travis-ci/travis-ci/issues/9069#issuecomment-425720905

swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy