Use stackato-buildpack-perl instead of the legacy buildpack

* remove framework setting from stackato.yml
* add buildpack reference to stackato.yml
* create cpanfile and cpanfile.snapshot
* delete requirements.txt
* update README to use carton instead of cpanm

Also add "ignores:" to stackato.yml and .gitignore file to deal
with running `carton install` and `carton bundle` locally.