www/serendipity: Upgrade from 2.0.2 to 2.0.4

• https://blog.s9y.org/archives/269-Serendipity-2.0.4-and-2.1-beta2-released.html
• https://blog.s9y.org/archives/266-Serendipity-2.0.3-released.html

PR: 213968
Submitted by: Jochen Neumeister <joneum@bsdproject.de>
MFH: 2016Q4