Speed up builds by only running against MS once

Ends up like this:

![image](https://cloud.githubusercontent.com/assets/36432/5174130/69c4de9c-73e3-11e4-9459-50e8ef109d7a.png)

I'd argue we really need one build per PHP version, and we could probably skip 5.4.

@WP-API/amigos opinions?