Upgraded to use and auto-ref NuGet.Core 2.5 in fake boot

1. Updated FakeLib NuGet.Core dependency to use NuGet.Core 2.5.0 package (was: 2.3.x-alpha). AFAIK NuGet.Core is only being used by `fake boot` so far, but anyhow it is nice to stay current.

2. When generating automatic references file in `fake boot`, added an #r directive to include NuGet.Core 2.5.0 from the FAKE build folder before referencing FakeLib. This makes sure that NuGet API is 2.5.0 in your build scripts even when an older NuGet is present in the GAC.

Thanks.