Bump .NetFramework Version

[ry8806]

I was wondering if bumping the .Net Framework version (i believe the .exe that's downloaded when you install Scripty via NuGet is v4.0)
Would bumping this to v4.5 allow us to start referencing .NetCore .dlls?
Looking at the following grid: https://github.com/dotnet/standard/blob/master/docs/versions.md we might be able to start referencing .Net Standard 1.0

This looks like it might partially solve issue #50 for some users too?