Attempt to fix Travis build on 5.20 and 5.22

Attempt to fix Travis build on 5.20 and 5.22

AFAICT, what's happening here is as follows.

The Travis build starts by installing Catalyst::Devel, which depends on
Catalyst, which is in the Catalyst-Runtime dist. So the latest released
version of Catalyst::Runtime gets installed to make that happen. The
release in question has an unversioned dependency on List::Util (well,
Scalar::Util), so that dep is met by the Perl core. Then the Travis build
runs the Makefile.PL, which prompts the user to install a newer version of
List::Util. Nothing attempts to answer that prompt, so the Travis build
times out.

Newer versions of Perl satisfy the 1.45 List::Util dependency natively.
For older versions of Perl, it seems to be the case that something else in
the dependency graph requires a newer version of List::Util than is shipped
with that Perl, and the latest version is selected automatically. So only
5.20 and 5.22 are affected by this.

We therefore change the Travis build so that, once the Makefile.PL has been
run (thus creating MYMETA.json) we try again to install any missing deps
using cpanm. Hopefully this will work!