Add all shells to Travis CI configuration

[skral]

This adds all shells in Mac OS and Linux to the Travis CI configuration. Running selftest in every shell will increase our confidence in merging new code changes.

While setting up the configuration I noticed tests are breaking for sh on Linux and tcsh on Mac OS. I will try to take a look at why they are failing so we don't end up with a "build:error" badge right after the merge.

@nerdvegas I know you haven't been a big fan of Travis CI so far but I am sure we can figure out why the badge hasn't been all green all the time.

Let me know what you think of this PR.

[skral]

Doing a build for each shell makes sense to find errors easier but it took almost 17 minutes for the 8 builds to complete. Doing one Linux and one Mac OS build, with all shells each, took a little under 5 minutes.

What do others think? Getting a quick response is worth a lot but not knowing where the error happens isn't great.

I think I would go with the combined approach for now, enjoy the better performance and see if we like it.

[skral]

Ah, on a sidenote, did you disable Travis builds for PRs, @nerdvegas? I think it would definitely help to enable those because it runs PRs through its paces and signals if the changes made in the PR break the tests. This helps you decide if it's ok to merge and helps the developer to see that there is still some work to be done right away.

[skral]

I had to do a couple more updates to the Mac OS side of things because I noticed the build failed from time to time because of the before install scripts erroring out. Feels a lot more robust now.

[nerdvegas]

On this soon @skral, cheers.

[nerdvegas]

Marking ON HOLD as we're going to be using a GitHub Actions- based CI test suite (or possible Azure Pipelines, but I'm banking on GH). This is for the purpose of readiness for ASWF adoption.

[nerdvegas]

Closed as we're now using GitHub Actions: #750