No tests run on non-Intel architectures & no coverage measurement for IPv6

[sigiesec]

Travis no longer (since beginning of 2019) has any IPv6 support on the amd64 machines, including the sole coverage job we currently have. Therefore, we can't see the actual coverage of the IPv6-specific code.

OTOH, travis now has arm64, ppc64le and s390x machines, which do have IPv6 support. We should add jobs running on those machines, including at least one (additional) coverage job, so we can understand the actual coverage of IPV6 code and ensure it doesn't break.

NB: Maybe we can remove some other non-coverage job, which is otherwise similar to the existing coverage code, since it's unlikely that the coverage job works in cases the non-coverage job doesn't?

[sigiesec]

NB: In https://github.com/sigiesec/libzmq/tree/test-coverage-arm64, I added a job doing this, which works fine in terms of the coverage report: https://coveralls.io/builds/33254295 (coveralls automatically merges the two coverage reports for the same revision). For some reason, one test case seems to break on arm64 for some reason I still need to investigate.

[stale[bot]]

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