Run nixpkgs-lint in travis

[copumpkin]

Seems like it would be helpful to run it on the diff to give people early feedback about common issues.

[matthewbauer]

There's a lot of warning on in master, so we may need to fix those first.

[copumpkin]

The ideal would be for the linter to only warn about lines you've touched, but that seems tricky.

Then we can whittle down the overall number of lint errors and also make it hard to add new ones.

[cmfwyp]

A simple solution is to check only the files modified. The list of files modified can be found with git diff --numstat HEAD~1. This means nixpkgs-lint needs to accept an argument for a file to only check that file instead of checking all of Nixpkgs.

Also, this might be a duplicate of #11265.

[copumpkin]

Oh, it is a dupe, sorry. I'll close!