Travis: explicitly use Ubuntu Trusty for builds
Matthieu Muffato's experiments with ensembl-hive have shown that in spite of https://docs.travis-ci.com/user/languages/perl/ claiming Travis supporting perl-5.8 and newer, the oldest version supported under the Xenial environment is 5.22. This has since been confirmed by a post on Travis community forums: https://travis-ci.community/t/failure-with-perl-5-16-5-18-5-20/2458 . We are presently not ready to drop support for Perl versions older than what Travis makes available under Xenial, therefore let us for the time being make sure we continue to use Trusty for now.
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