Update travis-ci integration to use docker containers

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Take a look at https://docs.travis-ci.com/user/docker/ for some info on how to do this.

[lobabob]

Some useful resources to look at when setting this up:

Official Docs:

• https://docs.travis-ci.com/user/docker/
• https://docs.travis-ci.com/user/build-stages/
• https://docs.travis-ci.com/user/caching/#Things-not-to-cache

Third-Party Links with useful info or examples:

• https://medium.com/mobileforgood/patterns-for-continuous-integration-with-docker-on-travis-ci-71857fff14c5
• https://stackoverflow.com/questions/35305492/cache-docker-images-on-travis-ci