Allow for cpanfile as well as Makefile.PL.

This PR allows using a [cpanfile](http://search.cpan.org/~miyagawa/Module-CPANfile-1.0002/lib/cpanfile.pod) instead of Makefile.PL to declare dependencies. Since the buildpack installs with `cpanm --install-deps` anyway, the `cpanfile` works exactly the same (as would `Build.PL` or `META.json`...I could add support for those too if you're interested).