Use pre-built Docker image

[mikeanthonywild]

The build time can be significantly reduced if we pull a pre-built image from Docker Hub. make shell should pull an image from Docker Hub by default.

PRs should use a pre-built image, however when Travis builds master we should re-build the image first and push it to Docker Hub.

All of this should be transparent as it is all abstracted by the Makefile.

• [x] Set up Docker Hub repository
• [x] Modify Makefile to pull image from Docker Hub
• [ ] Modify travis.yml to push new image when commit is made to master
• [ ] Update documentation

[mikeanthonywild]

On second thoughts, let's split out the image into its own repo. This repo should always be using a pinned version of the image.