See #51 for context.

With our Python runtime (debian8) using both certifi.where() and OS certs work with SNI. (This is what we do by default in _make_default_http)

With Travis (Ubuntu 12.04 LTS) using certifi.old_where() works but generates an InsecurePlatformWarning. Using OS certs works but generates an SNIMissingWarning. Using OS certs with PyOpenSSL works without warnings.