www/mybb: Update to 1.8.13

Changelog:
https://blog.mybb.com/2017/11/07/mybb-1-8-13-released-security-maintenance-release/

Approved by: tcberner (mentor)
MFH: 2017Q4
Security: 7761288c-d148-11e7-87e5-00e04c1ea73d
Differential Revision: https://reviews.freebsd.org/D13231