Use openjdk10 in travis build

Travis started giving this error:

oraclejdk10 is deprecated.
See https://www.oracle.com/technetwork/java/javase/eol-135779.html
for more details. Consider using openjdk10 instead.

.travis.yml

@@ -2,7 +2,7 @@ dist: trusty
 language: java
 jdk:
   - oraclejdk8
-  - oraclejdk10
+  - openjdk10
   - openjdk11
 env:
   global: