Travis: Replace Java 14 with 15.
diff --git a/.travis.yml b/.travis.yml
index da4d628..4f4f04f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,7 +22,7 @@
   include:
     - jdk: openjdk8
     - jdk: openjdk11
-    - jdk: openjdk14
+    - jdk: openjdk15
     - jdk: openjdk-ea
   allow_failures:
     - jdk: openjdk-ea