[PSR-7] Add MessageInterface::setProtocolVersion()

Per [discussion on the mailing list](https://groups.google.com/d/msgid/php-fig/cc4e5a30-f87d-4ce6-bbaa-8d86c10959db%40googlegroups.com), this patch adds a `setProtocolVersion()` method to the `MessageInterface`. Considering every other property of the message has both accessors and mutators, the lack of one for the protocol version was assymetric.