Replace Java 15 with Java 16 in Travis CI.
diff --git a/.travis.yml b/.travis.yml
index 4f4f04f..c48065a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,7 +22,7 @@
   include:
     - jdk: openjdk8
     - jdk: openjdk11
-    - jdk: openjdk15
+    - jdk: openjdk16
     - jdk: openjdk-ea
   allow_failures:
     - jdk: openjdk-ea