ci: travis: allow for some failures in the matrix build

ci: travis: allow for some failures in the matrix build

We want to make sure to have a stable and reliable subset of builds that
define if the build passed or not.
We also want to have builds which are more experimental. They give us a
good insight, but we are not yet ready to have them supported officially
as need-to-pass build. Namely the macOS build.

Another side effect of this change is that we reduce the critical build
matrix to 5 builds. The exact number of parallel ones we are allowed. With
fast_finish Travis will not wait for the other ones to finish before
setting the build status. This will allow us to have all builds in
parallel and not waiting for build #6 to be finished.