.travis.yml: specify "precise" Linux distro for JDK8 build
.travis.yml: specify "precise" Linux distro for JDK8 build

Travis's default Linux distro is now "xenial" !
jMonkeyEngine/jmonkeyengine,zzuegg/jmonkeyengine,jMonkeyEngine/jmonkeyengine,zzuegg/jmonkeyengine,jMonkeyEngine/jmonkeyengine,zzuegg/jmonkeyengine,zzuegg/jmonkeyengine,jMonkeyEngine/jmonkeyengine