Use latest pyOpenSSL for all travis tests environments

SSLv2 was removed from OpenSSL 1.0 and above but it is still referenced
by pyOpenSSL < 0.13. Travis workers are precise hosts with OpenSSL 1.0
and pyOpenSSL 0.12 (!) with a debian patch to workaround this problem
that is not present in pyOpenSSL 0.12 shipped by PyPi.

Trying to install pyOpenSSL 0.10 or 0.12 from packages at PyPi under a
system with OpenSSL >= 1.0 will success but fails at import time with a
message similar to:

    ImportError: .../lib/python2.7/site-packages/OpenSSL/SSL.so: undefined symbol: SSLv2_method