force tls 1.2 in soledad client. Partially fixes #6437

Force tls 1.2 in client. 
By default, this was using SSLv23 (We ARE NOT using the harcoded SSLv3 that's in u1db wrapper, which has been patched in debian).

I think we could cherry pick this (and probably the related server side fix) into a 0.6.x release branch, and tag a 0.6.1 version on time for platform release.

@andrejb whatchathink?