UnitsNet installs System.ValueTuple into net47 or higher targets

[tmilnthorp]

Describe the bug
UnitsNet installs System.ValueTuple into net47 or higher targets

To Reproduce
Steps to reproduce the behavior (just an example):

1. Add nuget UnitsNet 4.9.0 to a .NET 4.7 or higher project
2. See it installs System.ValueTuple

Expected behavior
.NET Framework 4.7 and higher has ValueTuples include in mscorlib