OAK-8894: Build error on travis-ci

Use trusty build image

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/branches/1.0@1873654 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/.travis.yml b/.travis.yml
index 27e6118..f753032 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,6 +14,7 @@
 # limitations under the License.
 
 sudo: false
+dist: trusty
 env:
   global:
     - MONGO_VER=2.6.12