Require IPv6 tests in Travis CI

While we're actively changing our IPv6 code, we should make sure that the macOS chutney IPv6 tests pass in Travis CI.

While I'm doing this change, I will see if there are:

some redundant jobs I can delete, or
some inactive areas that I can make fast_finish.

I think our Rust build might be a good candidate for fast_finish, we haven't changed that code much in about a year. But I should check with the team before making this change.

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