Specificaly choose Trusty for the Travis Host (#4168)

Signed-off-by: Jan Supol <jan.supol@oracle.com>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index b102f62..863c1e0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,5 @@
+dist: trusty
+
 sudo: required
 
 language: java