Remove the Travis CI config file.

Since the CI is now in GitLab.

.travis.yml

-# vim:ff=unix ts=2 sw=2 ai expandtab
----
-language: python
-python: "3.6"
-dist: bionic
-cache:
-  pip: true
-  directories:
-    - $HOME/.cache/pre-commit
-
-install:
-  - pip install pre_commit
-
-script:
-  - pre-commit run --all-files
-
-notifications:
-  on_failure: never
-  email: false