Remove explicit dependency reference to NetStandard.Library 2.0.0

Fixes #26

Author: reisenberger

src/Polly.Caching.Memory.nuspec

@@ -52,7 +52,6 @@
         <dependency id="Microsoft.Extensions.Caching.Memory" version="1.1.2" />
       </group>
       <group targetFramework="netstandard2.0">
-        <dependency id="NETStandard.Library" version="2.0.0" />
         <dependency id="Polly" version="6.1.1" />
         <dependency id="Microsoft.Extensions.Caching.Memory" version="2.0.2" />
       </group>