This commit removes a reference to the minimum required PHP version (which is presumably outdated as it mentions PHP 5.6) from the MailPoet API documentation.
I'm assuming that the minimum required version to use the API is the same as the one to run MailPoet. If that is the case, I don't see a reason to mention it in the API documentation, and removing it has the benefit of reducing the maintenance burden when the minimum required version is updated.
