Require highest level of client-supported SSL/TLS crypto

Up to TLS 1.2, support the highest locally supported crypto
protocol. This eliminates the previous default of PROTOCOL_SSLv23,
replacing it with what should be TLSv1 for Python 2.7 and TLSv1.2
for newer versions of Python 3.

Developers using docker-py may still specify the ssl_version to
override the default.

Fixes #367