bpo-39035: travis: Update image to xenial (#17623)

methane · web-flow · commit f88b578949a0 · 2020-06-12T07:46:40.000-07:00
Use image same to master to ease maintainance.
Remove "group: beta" to make Travis more stable.
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,5 @@
 language: c
-dist: trusty
-sudo: false
-group: beta
+dist: xenial
 
 # To cache doc-building dependencies.
 cache: pip
