Remove Travis support

I've just had it with Travis. I coped with it being extremely buggy and
basically every single build needing to be restarted a few times until
it works because their macOS machines (but not only those) are super
buggy. But then they got even more buggy and it was no longer possible
to restart builds - which meant it became pretty useless, as every 2nd
build failed because Travis was buggy and there was no way to restart
it.

I contacted support about this, but they were unwilling to help me at
all until I switch plans and continuously pressured me to do so. After
repeatedly being told that switching plans would fix all my problems, I
finally gave in and switched to the open source plan. Hurray, I could
finally restart the failed builds! Only to then notice: After just 2
builds(!!), they decided that that used up all my credits. I contacted
support and told them that following their repeated nagging to change
plans only made it worse and now I cannot do any builds at all anymore.
I was basically told "Tough luck, we might refill credits in the future,
but it's currently unimplemented". They did not disclose this fact to me
when they coerced me to switch plans before "helping" me.

It is clear by now that Travis isn't interested in the open source
community at all anymore. They exploited the open source community to
get big so that companies would buy their services and now that they
did, they no longer care about open source. Given this, they no longer
deserve the support of the open source community and given how painful
it always was to support them due to their extreme bugginess and not
having any way to test changes before committing them, I am no longer
willing to invest my time into this to give them free advertising by
making them able to say how many open source projects use them, and by
familarizing developers with their product by exposing developers to it
in open source projects, giving Travis an advantage over competitors.

I will look for alternatives, and I advise everybody reading this to do
the same.