update validation to allow sending "Bcc" and "Cc" without "To"

[daniel-keller]

Postmark now allows the sending of "Bcc" and "Cc" but the postmark-php validation forces a "To" address to be set.

[elynnaie]

The commit on my fork fixes this issue:

https://github.com/denaje/postmark-php/commit/641e73f3fcac10784271d83825a762a0846b20b4