Auto merge of #281 - servo:windows-ci, r=Manishearth
Enable AppVeyor.
<!-- Reviewable:start -->
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-mozjs/281)
<!-- Reviewable:end -->
