Merge pull request #1300 from gevent/travis-updates

Drop `sudo` from our Travis config, following current recommendation

.travis.yml

 # .travis.yml based on https://github.com/DRMacIver/hypothesis/blob/master/.travis.yml
 language: python
-sudo: false
-
+# We have some unexpected failures on xenial
+# dist: xenial
+# group: travis_latest
 python:
   - 3.7-dev