Wrong NETStandard.Library depedency in nuspec for NET45

[Hinni]

Dear Seq-Team

NETStandard.Library >= 1.6.0 depedency in nuspec for NET45 is wrong.
This depedency is not necessary for Tavis.UriTemplates in .NET 4.5 context.
It forces VisualStudio to download many NuGet packages which are not necessary.

Please remove it. Thank you.

[nblumhardt]

Thanks for the note. I've opened #23 to address this.