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?
Hum, it seems that the availabitily of TLS version 1.2 depends on the version of python itself. In the [ssl module doc](https://docs.python.org/2/library/ssl.html#ssl.PROTOCOL_TLSv1_2), it says "New in version 2.7.9". But my local python 2.7 available versions are 2.7.3 (wheezy) and 2.7.8 (sid).

It's weird, because the protocol version seems to be available in the bundle in the `OpenSSL` module but not in the `ssl` module:

    ipdb> import ssl
    ipdb> ssl
    <module 'ssl' from '/tmp/Bitmask-linux64-0.7.0rc5/./lib/ssl.py'>
    ipdb> ssl.PROTOCOL_TLSv1
    3
    ipdb> ssl.PROTOCOL_TLSv1_2
    *** AttributeError: 'module' object has no attribute 'PROTOCOL_TLSv1_2'
    ipdb> from OpenSSL import SSL
    ipdb> SSL
    <module 'OpenSSL.SSL' from '/tmp/Bitmask-linux64-0.7.0rc5/./lib/OpenSSL/SSL.py'>
    ipdb> SSL.TLSv1_METHOD
    4
    ipdb> SSL.TLSv1_2_METHOD
    6

Anyway, I think we're bound to the version available in server side, is that correct? Which should be TLSv1 because the server is wheezy. Just for the record, [this pull request](https://github.com/leapcode/soledad/pull/184) implements TLSv1 (no 1.2!) enforcement in server side.

How should we proceed with this? Maybe change this pull request to enforce TLSv1 instead of TLSv1_2?