.travis.yml: Use pre-commit for linting

.travis.yml

@@ -7,12 +7,12 @@ cache:
   pip: true
 
 install:
-- python -m pip install flake8 setuptools wheel
+- python -m pip install pre-commit setuptools wheel
 - python -m pip install -r requirements.txt
 - python setup.py install
 
 before_script:
-- flake8 --statistics .
+- pre-commit run --all-files
 script:
 - mkdir html
 - sphinx-multiversion docs html