Disable building on JDK 13 in CI
This commit disables building on JDK 13 in CI. The reason for this is
because Gradle is not yet ready to run on JDK 13. We could re-introduce
infrastructure to enable Gralde to run on a different JDK than the build
JDK, but rather than introducing such complexity we will instead wait
for Gradle to be ready to run on JDK 13.
Jason Tedor