Drop dotnetcore support down netstandard1.1

[Mpdreamz]

Since #2149 we built against netstandard1.3 for maximum platform coverage we should consider dropping down to netstandard1.1. Our old DNX based dotnet core beta build against dotnet5.1 which should be equivalent to netstandard 1.1.

A huge benefit of netstandard is that its self documenting and we do not need to get into the PCL framework matrix mess (#671)

[gmarz]

Will add that we're currently blocked from dropping lower than netstandard1.3 by the following dependencies

• System.Collections.Specialized
• System.Reflection.TypeExtensions

[Mpdreamz]

Closing until we see customer requests for this.