Clean-up: Have a dependency-only image hosted on docker hub

[MilesCranmer]

Since the dependencies for Bifrost have remained constant for a while, it seems like it would be more efficient to have a Dockerfile containing just the dependencies (maybe mirrored in another branch so you can pull from docker hub with docker pull ledatelescope/bifrost:deps).

The main Dockerfile would then just have to contain FROM ledatelescope/bifrost:deps, which would make it shorter, and would also mean that local builds would happen faster (you could just pull the dependencies and go). This would make the travis-ci tests faster too, as travis wouldn't have to keep re-building the entire dependency stack.

Docker hub has auto-building functionality on each commit, so we would still see the most up-to-date binaries being pulled.

[benbarsdell]

I've been working on some Dockerfile refactoring along these lines that I'll push soon.

[benbarsdell]

See PR #92