Try testing on Bionic again. Fixes #1623

.travis.yml

@@ -7,7 +7,7 @@
 # python: one day so making the migration simpler). "shell" and
 # "minimal" are aliases.
 language: shell
-dist: xenial
+dist: bionic
 group: travis_latest
 os:
   - linux

docs/changes/1623.feature

+gevent's CI is now tested on Ubuntu 18.04 (Bionic), an upgrade from
+16.04 (Xenial).