Allow configuration of ssl version

As of the release of openssl 1.0 talking to some servers via SSL results in the connection being reset. This has something to do with how the new version of openssl negotiates which version of the SSL protocol to use. There needs to be a way to force the SSL version that active_merchant / active_utils uses. This commit takes care of that.

The issue with SSL can be seen in more detail here: http://stackoverflow.com/questions/11321403/openssl-trouble-with-ruby-1-9-3/13811555#13811555