Replace Java 15 with Java 16 in Travis CI.
diff --git a/.travis.yml b/.travis.yml
index 8cd3061..ff77cac 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -19,10 +19,10 @@
   include:
     - jdk: openjdk8
     - jdk: openjdk11
-    - jdk: openjdk15
+    - jdk: openjdk16
     - jdk: openjdk-ea
   allow_failures:
-    - jdk: openjdk15
+    - jdk: openjdk16
     - jdk: openjdk-ea
 after_success:
   - mvn -V --no-transfer-progress -B -e clean cobertura:cobertura coveralls:report