Replace Java 15 with Java 16 in Travis CI.
diff --git a/.travis.yml b/.travis.yml
index 6be9ce7..16387c3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -19,7 +19,7 @@
 jdk:
   - openjdk8
   - openjdk11
-  - openjdk15
+  - openjdk16
 
 script:
   - mvn -V --no-transfer-progress -Ddoclint=all