bpo-36307: Travis: upgrade to Xenial environment (GH-12356)

(cherry picked from commit 74ae50e5)
Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>

.travis.yml

 language: c
-dist: trusty
-sudo: false
+dist: xenial
 group: beta
 
 # To cache doc-building dependencies and C compiler output.