Updated .travis.yml.

Updated .travis.yml to use the Ubuntu 16.04 (Xenial) build environment.
In this environment perl versions before 5.22 is not available, see
https://travis-ci.community/t/failure-with-perl-5-16-5-18-5-20/2458

Also from Perl 5.22, Module::Build is not in core, see:
https://metacpan.org/pod/release/RJBS/perl-5.22.0-RC2/pod/perldelta.pod
so added that as a before_install dependency in .travis.yml.

.travis.yml

@@ -1,12 +1,13 @@
 language: perl
 perl:
-  - "5.12"
-  - "5.14"
-  - "5.16"
-  - "5.18"
-  - "5.20"
+  - "5.22"
+  - "5.24"
+  - "5.26"
+  - "5.28"
+  - "5.30"
 
 before_install:
-  cpanm -n Devel::Cover::Report::Coveralls
-script:
-  perl Build.PL && ./Build build && cover -test -report coveralls
+  - cpanm -n Module::Build
+  - cpanm -n Devel::Cover::Report::Coveralls
+
+script: perl Build.PL && ./Build build && cover -test -report coveralls