Add --author flag to `dzil listdeps' in .travis.yml.

At the some point, dependent modules for some author/release tests included in the Dist::Zilla bundle become NOT optional:
https://github.com/rjbs/Dist-Zilla/commit/b1eac573e8e2e0271e18aba27633e9c548c1da6f

On the other hand, the dependencies are added only as develop phase:
https://github.com/rjbs/Dist-Zilla/blob/master/lib/Dist/Zilla/Plugin/PodCoverageTests.pm#L28

So, `dzil listdeps` is not enough to make sure to install dependencies for author/release tests.

By the changes of this pull request, the following failure:
https://travis-ci.org/yak1ex/String-Unescape/jobs/24317647#L225
is recovered as:
https://travis-ci.org/yak1ex/String-Unescape/jobs/24319839#L311

(Please ignore failure of t/release-kwalitee.t: 22. To tell the truth, it is related with the DZ changes described at the beginning of this comment, but not related with Dist-Zilla-TravisCI)