Fedora 27 and LLVM 5 are EOL, which means they won't receive new
updates (not even security patches). As such, there's no reason to keep
testing those platforms/dependencies in our CI, and removing them will
reduce our CI complexity as well as unbreak our CI.

Also, static builds are broken for some time
(https://github.com/iovisor/bpftrace/issues/266), and keeping them in
our CI will only prevent collaborators from keeping the CI healthy. We
should push for Ubuntu/Debian/Fedora static builds instead of relying on
alpine for it. For now, mark alpine static build as "allow to fail" on
Travis.