Restrict build system to 'trusty'

... so that the relevant Perls can be installed.  Travis-CI doesn't
build old Perls for Xenial (Ubuntu 16)[1], hence the builds from 5.20
and below fail because the system Perl is used and not a perlbrew-ed
Perl.  This commit should get Travis builds for these old versions back
working again.

[1] https://travis-ci.community/t/failure-with-perl-5-16-5-18-5-20/2458

.travis.yml

@@ -1,5 +1,7 @@
 sudo: true
 
+dist: trusty
+
 language: perl
 
 perl: