Travis CI timeouts for MacPorts builds

[Ralph Seichter]

* Joshua Root:

>> [1] https://travis-ci.org/github/macports/macports-ports/jobs/724689780
>
> It's just a matter of how long your port takes to build (including
> installing all its dependencies).

Notmuch, which is what was built in the job [1], is small and builds in
less than a minute on my MacBook once the prerequisites are installed.
Perhaps you'd like to try it for yourself.

Is the Travis-based build just installing the dependencies (as in: the
binary packages), or does it also build the dependencies from source?
I suspect the latter. I actually tried that here, and it took ages,
even on my octa-core machine.

-Ralph