Merge pull request #5 from LorenzoBettini/travis-linux-dist

explicit Travis Linux distribution

.travis.yml

@@ -3,10 +3,14 @@ language: java
 matrix:
    include:
       # always build this environment
-      - jdk: openjdk8
+      - os: linux
+        dist: xenial
+        jdk: openjdk8
         env: ADDITIONAL_MAVEN_ARGS="-Pjacoco coveralls:report"
       # only on PR
-      - jdk: openjdk9
+      - os: linux
+        dist: xenial
+        jdk: openjdk9
         if: type IN (pull_request)
       # only on PR or after merging a PR
       - os: osx