TAMAYA-277: Add Trusty as CI environment

Maybe this helps to get the sonar integration up and running.
diff --git a/.travis.yml b/.travis.yml
index 0b865de..e5ec8ef 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,5 @@
 language: java
+dist: trusty
 jdk:
   - openjdk8
   - openjdk9