Update Travis CI.

.travis.yml

+# vim:ff=unix ts=2 sw=2 ai expandtab
 ---
 language: python
 python: "3.6"
-dist: trusty
-sudo: false
-group: beta
+dist: bionic
 cache:
-  - pip
-  - directories:
-      - $HOME/.cache/
+  pip: true
+  directories:
+    - $HOME/.cache/pre-commit
 
 install:
-  - pip install pre_commit | cat
+  - pip install pre_commit
 
 script:
   - pre-commit run --all-files