Travis integration, conda environment yml

[JelleAalbers]

This does the following:

• Add a .travis.yml file to test the container build on TravisCI. Besides testing, we could use Travis in the future to push new container releases automatically (https://docs.travis-ci.com/user/docker/#pushing-a-docker-image-to-a-registry).
• Move the list of conda packages to a conda.yml file. For clarity I listed only the conda packages we need directly for the moment, though I understand we might want to list all of them for maximum reproducibility. Pyup can apparently keep these yml files up to date just as it does for requirements.txt. If this would work, we can pin our dependencies here and remove the functionality in strax (see setup.py should read from a minimal-requirements list by default AxFoundation/strax#197)
• Add code to run the strax/straxen tests at the end of the container build. If the tests fail, the container exits.
• Add installation of the tensorflow2 beta, used in flamedisx and the new straxen version. I only got this to work by first installing regular tensorflow via conda (which pulls in a lot of related packages), then installing tensorflow2 via pip. This should become easier once tf2 is officially released, but that could be a few months.
• Replace anaconda by miniconda. This means the base environment is lighter. Maybe we can also move to miniconda3, so the base environment is also python 3 (although it doesn't matter much since the base env is not used).

Currently the build fails (see https://travis-ci.org/JelleAalbers/base_environment) due to an issue with the GFAL2 python bindings: they can't find the boost headers. I saw the same problem trying to build the container on my laptop, though I don't think I tried exactly the current master (and I don't have enough disk space at the moment to try again ;-). @rynge Is the current master working for you, and if so, any ideas on what I messed up?

The travis.yml file has a curious while command, borrowed from travis-ci/travis-ci#4190. We currently compile boost from source, which takes >= 15 minutes. This is longer than TravisCI allows commands to run without producing output. I tried adding boost to the conda package list instead, but then gfal complains about missing headers. I tried to remove run_quietly, but then ravis kills the job for producing too much output. I also tried travis_wait, but this apparently only works when wrapped around the entire build command, and would suppress its output.

(and sorry for the mess of commits, when we get this to work you probably want to squash-merge)

[rynge]

@JelleAalbers Thanks! This looks great. Re boost - let me work on this and see if we can't pull boost from anaconda as well. Note that I already have deployments in DeployHQ, but switching to Travis makes sense if that is what the rest of the project is using. I will try to look at this later this week.

[rynge]

I'm going to merge this, and then put my local changes on top