refs and cleanup

Author: SimonCropp

src/Directory.Packages.props

@@ -10,7 +10,7 @@
     <PackageVersion Include="NodaTime" Version="3.2.0" />
     <PackageVersion Include="NUnit" Version="4.2.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
-    <PackageVersion Include="Polyfill" Version="7.4.1" />
+    <PackageVersion Include="Polyfill" Version="7.5.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.141" />
     <PackageVersion Include="System.Collections.Immutable" Version="8.0.0" />
     <PackageVersion Include="System.IO.Compression" Version="4.3.0" />