Upgrade Umbraco dependencies from nightly to release

I had trouble building the project as NuGet couldn't find the package:  UmbracoCms.Core 7.1.5-nightly211

To get the build working I removed this and installed Umbraco's 7.1.5 NuGet packages.  Not sure if there was a reason for referencing the other one, but thought I'd send this in case :+1:

This includes binary updates as well as content updates for the TestSite