Use docker in travis ci perl buildsRecently, the default ubuntu version in travis ci has changed from trusty (14.04) to xenial (16.04). This may cause test failures for some perl projects because old perls are not available for xenial. And it seems that it is intentional https://travis-ci.community/t/failure-with-perl-5-16-5-18-5-20/2458. To be fair, I think it is reasonable they does not support old perls. So if you still want to
