Travis CI: Drop Java 12 build (Java 13 is now Generally Available).
diff --git a/.travis.yml b/.travis.yml
index 1e3de86..155fd53 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -18,7 +18,6 @@
 jdk:
   - openjdk8
   - openjdk11
-  - openjdk12
   - openjdk13
   - openjdk-ea