[ssl] add certifi as source of root CA

Certifi is a collection of root CA certificates, extracted from requests project.
It's easy to use, and allows package maintainers to focus on more important things instead of bringing their own certificate collection (or trying to find system's collection).
It should help this project with:
 - Handling SSL errors originated by outdated root CA certificate collection
 - Handling SSL errors due to missing certificate collection file
 - Supporting wider range of platforms out of the box (OS X for example)