adds AppVeyor 2 beta support

For #473, adds AppVeyor 2 beta support. You can see it working here. You can also copy the nuget link and try out the nupkg that was created.

https://ci-beta.appveyor.com/project/ctaggart/fsharp-data

![image](https://f.cloud.github.com/assets/80104/2303609/d0444db2-a1d2-11e3-89ec-b8abe8db5e2a.png)

![image](https://f.cloud.github.com/assets/80104/2303610/d461b614-a1d2-11e3-9e0d-82e364fb42ff.png)