Remove dependency on System.ValueTuple NuGet package when targeting .Net 4.7

[NightWatchman]

System.ValueTuple is incompatible with .Net Framework 4.7 (because it's already built in to 4.7). If forced to reference the System.ValueTuple nuget package, which is brought in by MoreLINQ as a package dependency causes a conflict and prevents the use of tuple language features available in .Net 4.7.

I see you're already multi-targeting and have provisions in your project file. It just needs an adjustment for net47

[atifaziz]

This is a duplicate of #280.