Add `cpanminus` software definition

This fast, modern CPAN client allows a greater level of configuration
than the default `cpan`. The project describes itself as follows:

“Cpanminus is a script to get, unpack, build and install modules
from CPAN and does nothing else. It's dependency free (can bootstrap
itself), requires zero configuration, and stands alone. When running, it
requires only 10MB of RAM.”

More info here:
https://github.com/miyagawa/cpanminus#questions

This is also the CPAN client Travis CI uses:
http://docs.travis-ci.com/user/languages/perl/#Dependency-Management
Use `cpanminus` to install Perl module deps

We’ll also ensure we don’t run tests when installing CPAN modules. Go
read this if you need a reason why this is insane:
http://www.openswartz.com/2012/01/31/stop-running-tests-on-install/

We’ve hit issues where a modules test’s fail (and thus the module is NOT
installed) and the default `cpan` client continues along and exits with a
zero status!
Add a real version/md5 to `perl_pg_driver`