Do not lock SMTP STARTTLS to only use SSLv3

SSLv3 has been deprecated for some time, and is being actively disabled more
and more (http://disablessl3.com, https://tools.ietf.org/html/rfc7568).

To maintain forward compatibility, do not specify a maximum version
and insteady use the default from the local OpenSSL library instead. Fallbacks
to older versions will happen on handshake as needed.

lib/msf/core/exploit/smtp_deliver.rb

@@ -229,7 +229,7 @@ protected
   end
 
   def generate_ssl_context
-    ctx = OpenSSL::SSL::SSLContext.new(:SSLv3)
+    ctx = OpenSSL::SSL::SSLContext.new
     ctx.key = OpenSSL::PKey::RSA.new(1024){ }
 
     ctx.session_id_context = Rex::Text.rand_text(16)