add travis

.travis.yml

@@ -0,0 +1,36 @@
+language: java
+
+dist: xenial
+
+before_cache:
+  - rm -rf $HOME/.m2/repository/io/jenkins/plugins/gitlab-branch-source*
+
+cache:
+  directories:
+    - $HOME/.m2
+
+env:
+  global:
+    - JAVA_LEVEL=8
+
+matrix:
+  include:
+    - jdk: openjdk11
+      env:
+        - MVN_FLAG='-Djenkins.version=2.164.1 -Daccess-modifier-checker.failOnError=true'
+    - jdk: openjdk8
+      env:
+        - MVN_FLAG='-Djenkins.version=2.164.1 -Daccess-modifier-checker.failOnError=true'
+    - jdk: openjdk8
+
+install:
+  - mvn clean install -DskipTests=true -Dmaven.javadoc.skip=true -B -V $MVN_FLAG -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn
+
+script:
+  - |
+    mvn clean install -B \
+      -Djava.level=$JAVA_LEVEL \
+      $MVN_FLAG -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn
+
+notifications:
+  email: false