Update Travis CI Linux distro to 16.04 (Xenial)

Change-Id: I74638b2defd2992304fcf64fc2be5b2c07ad247f

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index d159316..7d14971 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,12 +4,12 @@
   fast_finish: true
   include:
   - os: linux
-    dist: trusty
+    dist: xenial
     sudo: required
     compiler: gcc
     env: CERES_BUILD_TARGET=LINUX
   - os: linux
-    dist: trusty
+    dist: xenial
     sudo: required
     compiler: gcc
     env: CERES_BUILD_TARGET=ANDROID