There's an option for that, but it only allows specifying SSL2, SSL3, and TLS1.  We'd need SSL3, TLS1, TLS1.1, and TLS1.2.  See
https://github.com/rapid7/metasploit-framework/blob/8566c1dad48b7cd0f8c7af9a3f51057f9b667a4e/lib/rex/socket/ssl_tcp.rb#L56

I'm not sure it matters all that much though, connect() appears to negotiate the version; the module only needs it specified since it's building the hello and heartbeat manually. (I'd like to fix that at some point so we can encrypt the heartbeat/bleed packets, but that'll likely require deeper changes to REX)