Document security issues fixed in CodeIgniter 3.1.0 and 3.1.2
Note the security issue in CodeIgniter 3.1.1 is documented being valid on
PHP 5.3 only.
Security: https://vuxml.FreeBSD.org/freebsd/5e439ee7-d3bd-11e6-ae1b-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/496160d3-d3be-11e6-ae1b-002590263bf5.html