Travis: use TOXENV=linting for linting stage

[blueyed]

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

See https://travis-ci.org/pytest-dev/pytest/jobs/420595379 for an example failure.

Appveyor uses TOXENV=linting already.

[coveralls]

Coverage increased (+0.05%) to 92.639% when pulling 415fcb9 on blueyed:improve-pre-commit into be4b359 on pytest-dev:master.

[asottile]

Change seems fine 👍

I wonder if this was done to avoid tox overhead though -- but that's, what, a few seconds?

[asottile]

oh actually, maybe add skip_install / skip_sdist / whatever the option I'm forgetting to the linting testenv? It seems to still be installing pytest which isn't necessary