travis: fail if any cask files added outside Casks dir

[jawshooah]

Refs #26791 (comment)

[BenjaminHCCarr]

Thank you!

As mentioned in #267915#issuecomment-260818620

I broke my own patch tree, and it looks like the patches were coming off / instead of /Casks.

Sincerely sorry for breaking Cask for everyone.

[jawshooah]

Accidents happen 😄

[miccal]

Hello @jawshooah, it seems the the builds are failing for all new casks. I think your lines 24 and 26 in your modified file ci/travis/script.sh shouldn't have the arithmetic tests arg -gt 0 but instead should have -n arg?

Edit: It is passing for modified Casks, so the problem is not what I thought.

[BenjaminHCCarr]

I'm seeing this in #26845 as well if you want an example......
cc: @jawshooah @reitermarkus @vitorgalvao @MikeMcQuaid

[jawshooah]

@miccal The arithmetic test isn't the problem; it's just checking if the array casks_outside_casks_dir is non-empty. Testing the length with -gt, -lt, etc is less error-prone than string tests like -n, -z, etc because they only operate on the first element of the array. See the answers here.

The problem is that git diff *.rb is pulling in files outside the current working directory. Using ./*.rb instead doesn't seem to change anything. Will investigate further later, but in the meantime this can be reverted.