This hack has been in Homebrew Cask for more than two years
(since 51f93e6dc9c3da4ab2118459ea95e45c104386ec), and it originated even
earlier (6d2f7bc55af0b2aa915b2396d213e30a4446256c).

Cask tests apparently aren't even run on Travis anymore,
so this can be safely removed.

See https://github.com/caskroom/homebrew-cask/issues/24519

Feel free to suggest alternative wording and such.

OS X Travis CI is particularly error-prone, slow and unnecessary for
us when we're already running (fast) OS X builds on Jenkins so use it
just for Linux testing.

This has been causing sporadic CI failures.

This fixes the ignoring in libgit2 clients that don't read nested
`.gitignore` files.