Create `cpanminus` software definition

This modern CPAN client allows a greater level of configuration than the
default `cpan`. In particular it allows test to NOT be run as modules
are being installed. We’ve hit issues where tests fail, a module is not
installed but the default `cpan` client still exits zero!

The project describes itself as follows:

“This is a modern CPAN client 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 also happens to be the default 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!