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Co-Authored-By: robbuckley <20515024+robbuckley@users.noreply.github.com>

To get pip's platform.

We can't easily predict what the output directory will be for a Github
release archive.  Get fetch_unpack to tell us.  Add some tests that
won't fail, but at least show us the output.

See failure at https://travis-ci.org/matthew-brett/multibuild/jobs/382687141#L3065

Skip build tests if no docker available.

(There is no docker available for precise, so skip the builds for
precise).

Specify platform with env var, test 32 and 64 bit builds.

Previous version was using a sub-shell to execute, and therefore not
catching errors during execution of the functions that had errors, but
did not return non-0 exit codes.

Add tests.

See: https://github.com/matthew-brett/multibuild/issues/166

Thanks to Andrew Murray for pointing out this bug.

* Don't update homebrew

Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116