[ssl] add certifi as source of root CA

Certifi is a collection of root ca certificates, extracted from requests project.
It's much easier to use it, instead of bring your own certificate collection.
It should help this project with:
- handling ssl errors origined by outdated root ca collection
- supporting wider range of platforms out of the box (os x for example)
[ssl] adjust root ca collection discovery
[web] tiny typo fix