Enable AppVeyor Testing

[ChrisRackauckas]

I think AppVeyor testing should be enabled on all of the JuliaDiffEq repositories in addition to Travis. Windows is a large set of users (myself included) and AppVeyor many times will catch Windows-specific issues (usually dealing with the way integers can be different, but also sometimes with how roundoff is handled or obscure facts that are hard to catch by yourself). This code is simple enough that I don't expect it to be different than the Travis results, but it's a very simple no-cost double-check and so I think it should be enabled.