enable Travis CI

.travis.yml

@@ -0,0 +1,19 @@
+language: java
+sudo: false
+
+jdk:
+  - oraclejdk8
+  - oraclejdk9
+  - openjdk11
+
+before_cache:
+  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/
+
+before_install:
+  - ./gradlew --version
+  - java -version