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?
It is also not available in my version of the `OpenSSL` module:

    >>> from OpenSSL import SSL
    >>> SSL
    <module 'OpenSSL.SSL' from '/usr/lib/pymodules/python2.7/OpenSSL/SSL.so'>
    >>> SSL.TLSv1_2_METHOD
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'module' object has no attribute 'TLSv1_2_METHOD'
    >>> SSL.TLSv1_METHOD
    4