Travis: use TOXENV=linting for linting stage

This will run it with `--show-diff-on-failure` then, and helps to keep
it in line / in a central place.

.travis.yml

@@ -62,12 +62,7 @@ jobs:
           repo: pytest-dev/pytest
     - stage: linting
       python: '3.6'
-      env:
+      env: TOXENV=linting
-      install:
-      - pip install pre-commit
-      - pre-commit install-hooks
-      script:
-      - pre-commit run --all-files
 
 script: tox --recreate