Set Trusty for the Travis to support OracleJDK8

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

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index c09cafe..e599ed8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,5 @@
+dist: trusty
+
 language: java
 
 jdk: