Support for OpenJDK 15

Signed-off-by: Markus KARG <markus@headcrashing.eu>
diff --git a/.travis.yml b/.travis.yml
index 937737e..e6089d0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,6 +13,7 @@
   - openjdk12
   - openjdk13
   - openjdk14
+  - openjdk15
 
 cache:
   directories: