[FIX] using Java9

SeeSharpSoft · SeeSharpSoft · commit 847ee212ad24 · 2020-04-26T20:50:38.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: java
 jdk:
-  - openjdk8
+  - openjdk9
 
 env:
   - IDEA_VERSION=PC-2019.3.3 GRAMMAR_KIT_VERSION=2019.3
