 Removed JDK13 compilation from travis build matrix (#864)
 Removed JDK13 compilation from travis build matrix (#864)

The entry will have to be added again when Gradle 6 becomes available.
fixes TweetWallFX/TweetwallFX#863
jmichelberger/TweetwallFX,mklaehn/TweetwallFX,svenreimers/TweetwallFX,TweetWallFX/TweetwallFX,fvogler/TweetwallFX,reinhapa/TweetwallFX