ci: Travis: use dist=xenial

.travis.yml

@@ -1,15 +1,15 @@
+dist: xenial
 language: python
-sudo: false
 python:
-  - 2.6
   - 2.7
   - 3.4
   - 3.5
   - 3.6
-  - pypy
+  - 3.7
+  - pypy2.7-6.0
+  - pypy3.5-6.0
 matrix:
   include:
-    - { python: "3.7", dist: xenial, sudo: true }
     - python: 3.5
       env: TOXENV=cov
   allow_failures: