Replace SSL 3 with TLS 1.2

With SSL 3 POODLE issue (CVE 2014-3566) `ssl_version` set to `:SSLv3` raises an exception:

    OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv3 read server hello A: sslv3 alert handshake failure

With `ssl_version` set to `:TLSv1_2` Box connection works as expected.