Use OpenJDK 8 for Travis docs builds
Use OpenJDK 8 for Travis docs builds

Per https://github.com/dita-ot/dita-ot/pull/3348

Already implemented twice in core repo:
- on hotfix/3.3.4 with dita-ot/dita-ot@c95e528
- on develop with dita-ot/dita-ot@ecf4ebd

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

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