Appveyor builds are broken

One of the integration tests for alternate backends [calls out to `tee`][t] to write `stdin` to a file. Because the `tee` program doesn't exist on Windows, appveyor builds are failing.

[t]: https://github.com/rust-lang-nursery/mdBook/blob/80f42675d6b72b64447d71fc140784ea96b6bba7/tests/alternate_backends.rs#L37