Stay explicitly on Trusty so that Perl 5.18 and below are available

Travis CI recently switched to Xenial, and with that
Perl 5.18, 5.16 and 5.10 build targets are no longer
available.

Unfortunately, even most recent macOS ships with Perl 5.18,
so to keep this build target, we need to stay on Trusty.

.travis.yml

@@ -1,4 +1,5 @@
 language: perl
+dist: trusty
 install:
  - cpanm -n -q --skip-satisfied --installdeps .
 before_script: