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?
In which `ssl` module version is this protocol version available? I don't have it here:

    >>> import ssl
    >>> ssl
    <module 'ssl' from '/usr/lib/python2.7/ssl.pyc'>
    >>> ssl.PROTOCOL_TLSv1_2
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'module' object has no attribute 'PROTOCOL_TLSv1_2'
    >>> ssl.PROTOCOL_TLSv1
    3