travis: test on xenial rather than trusty, see #5383

.travis.yml

@@ -9,11 +9,11 @@ matrix:
     include:
         - python: "3.5.2"  # Use version 3.5.2 because it has lzma support while 3.5(.0) does not
           os: linux
-          dist: trusty
+          dist: xenial
           env: TOXENV=py35
         - python: "3.6"
           os: linux
-          dist: trusty
+          dist: xenial
           env: TOXENV=py36
         - python: "3.7"
           os: linux