Fix Java memory errors with Gradle 5.2
Fix Java memory errors with Gradle 5.2

Adjust Gradle memory allocation per
https://docs.gradle.org/5.2/userguide/build_environment.html#sec:configuring_jvm_memory

> "Expiring Daemon because JVM Tenured space is exhausted"
> plugins/org.dita.pdf2.fop/build_fop.xml:153: java.lang.OutOfMemoryError: Java heap space

Signed-off-by: Roger Sheen <a61a1add71dadc74037ae5e44ee30713db9a7451@infotexture.net>

infotexture/docs,dita-ot/docs,infotexture/docs