Security update to 4.2.10

• Now requires p5-Encode >= 2.64 from the converters/p5-Encode port, as the version bundled with perl is too old.
• New dependency p5-HTML-FormatExternal

There are database schema updates to apply, but these only affect
people using MySQL

ChangeLog: https://bestpractical.com/release-notes/rt/4.2.10
Security Advisory: http://blog.bestpractical.com/2015/02/security-vulnerabilities-in-rt.html

MFH: 2015Q1
Security: d08f6002-c588-11e4-8495-6805ca0b3d42