It appears that the payment library has been abandoned (no updates in almost three years), and there have been some changes with things like Visa cards that can have up to 19 characters. Either the validation library needs to be replaced with something else or removed entirely.

My recommendation would be to remove the built-in validation and just provide some links in the documentation to a few validation libraries along with some demo code on basic usage. This way the dependency on an outdated library can be removed and make the resulting package smaller. I don't have a specific validation library in mind so I can't recommend one over another.