Travis: use "xenial" Linux dist to solve TLS issue (#1081)

Fixing the broken build. Thanks to @Ali-RS for providing the hint:  https://travis-ci.community/t/install-of-openjdk11-is-failing-again/3061/16

Author: stephengold

.travis.yml

@@ -15,7 +15,7 @@ matrix:
     env: UPLOAD=true UPLOAD_NATIVE=true
   - os: linux
     jdk: openjdk11
-    dist: precise
+    dist: xenial
   - os: osx
     osx_image: xcode9.3
     env: UPLOAD_NATIVE=true