Use bionic distribution instead of trusty

Downloading archive: ...binaries/ubuntu/14.04/x86_64/python-3.8.tar.bz2
$ curl -sSf --retry 5 -o python-3.8.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404 Not Found
Unable to download 3.8 archive. The archive may not exist.
Please consider a different version.

.travis.yml

@@ -1,4 +1,4 @@
-dist: trusty
+dist: bionic
 sudo: false
 language: python
 python: 3.8