travis: Use bionic (#424)

methane · web-flow · commit 4c1950148ba1 · 2020-02-17T16:01:43.000+09:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,7 @@
-dist: xenial
+dist: bionic
 language: python
 
-# See aws s3 ls s3://travis-python-archives/binaries/ubuntu/16.04/x86_64/
+# See aws s3 ls s3://travis-python-archives/binaries/ubuntu/18.04/x86_64/
 python:
   - "nightly"
   - "pypy3"
