"Mirror" commits to the `pipe-osx` repo, to use two `.travis.yml`s.

Travis' linter responds to the `os` key, but they don't do anything
with it yet.  Instead, let this repo use `language: c` (for a linux
build) and give `language: objective-c` to `pipe-osx`.  That repo
is just a submodule (back to this repo) and a custom `.travis.yml`
that runs `scripts/travis-osx`.
Initial AppVeyor build.

Essentially stolen from julia's Windows CI PR.  As there, we rely on
AppVeyor implicitly providing MSYS1 for a shell.