[CALCITE-2728] Update appveyor.yml to enable appveyor testing against JDK 11

Close apache/calcite-avatica#81
diff --git a/appveyor.yml b/appveyor.yml
index 09d78f8..eb44d05 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -34,6 +34,7 @@
     - JAVA_HOME: C:\Program Files\Java\jdk1.8.0
     - JAVA_HOME: C:\Program Files\Java\jdk9
     - JAVA_HOME: C:\Program Files\Java\jdk10        
+    - JAVA_HOME: C:\Program Files\Java\jdk11
 build_script:
   - mvn clean -V install -DskipTests -Dmaven.javadoc.skip=true -Djavax.net.ssl.trustStorePassword=changeit
 test_script: