Send texts individually

If we pass all the phone numbers at once in one email, it becomes
a group chat, and that allows the recipients to see each other's
number, which isn't the intended behavior.

lib/rex/proto/sms/client.rb

@@ -51,7 +51,9 @@ module Rex
           begin
             smtp.enable_starttls_auto
             smtp.start(helo_domain, username, password, login_type) do
-              smtp.send_message(message, from, recipients)
+              recipients.each do |r|
+                smtp.send_message(message, from, r)
+              end
             end
           ensure
             smtp.finish if smtp && smtp.started?