Use new function signature for cached downloader
Use new function signature for cached downloader

* Until it's necessary to customize the cert list, the buildpack downloader
  will only trust the certs that are already present on the system.

Signed-off-by: Jen Spinney <475b118f8db4f150e7ed3b69b35fd7c857f9c9f8@hpe.com>

cloudfoundry-incubator/buildpack_app_lifecycle,cloudfoundry-incubator/buildpack_app_lifecycle