Emailqueue v3.4.12

• Upgraded PHPMailer version dependency to avoid vulnerabilities.
• is_sent, is_cancelled and is_blocked database fields are not set to 0 by default. Thanks to @marco-pm for noting this problem. Issue #29
• Added php and mysqli to composer dependencies. Thanks to @marco-pm for suggesting this improvement. Issue #28
• Switched to semantic versioning. (From v3.412 to v3.4.12).