Support Server Name Indication (bug 875142)

requests 2.0.0 has SNI support if the correct dependencies are installed, and doesn't have the breakage with persona that 1.2.3 had. Unfortunately we can't directly use the latest, 2.0.1, because of https://github.com/kennethreitz/requests/issues/1732

See also the old PR from @kumar303, which we never merged because of the persona breakage: #888