Travis-CI: Switch to OpenJDK for 10, add 11

The oraclejdk10 target offered by travis is sometimes unstable due to
JDK download brittleness. We thus switch to the openjdk10
target. Additionally, add openjdk11 as a target.

.travis.yml

@@ -6,11 +6,12 @@ dist: trusty
 sudo: required
 language: java
 
-# we test for JDK8 and JDK9
+# we test for JDK 8,9,10,11
 jdk:
   - oraclejdk8
   - oraclejdk9
-  - oraclejdk10
+  - openjdk10
+  - openjdk11
   
 # compile and run the test suite
 # we manually set PRISM_JAVA to the java on the PATH as the python invocation of prism-auto fiddles with the PATH variable...