Suggest FOP version 0.20.4

Note name of DTD is "DocBook XML 4.2"


[SVN r18108]

tools/boostbook

@@ -1 +1 @@
-Subproject commit 969f017fc438dc87fa9098df7e7d972ec94978e1
+Subproject commit 71cef2a6260fedd7d25ba441d9aaf9f51e560266