travis: use openjdk8 and xenial (ubuntu 16.04) (#3466)

This fixes Travis to use openjdk8 and Xenial (Ubuntu 16.04) as base
distro.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Author: yadvr

.travis.yml

@@ -15,11 +15,11 @@
 # specific language governing permissions and limitations
 # under the License.
 sudo: required
-dist: trusty
+dist: xenial
 group: edge
 language: java
 jdk:
-- oraclejdk8
+- openjdk8
 python:
   - "2.7"
 cache: