Remove explicit statement of NetStandard.Library 2.0 dependency, for netstandard2.0 tfm

Fixes #9

Author: reisenberger

src/Polly.Caching.Distributed.nuspec

@@ -34,7 +34,6 @@
         <dependency id="Microsoft.Extensions.Caching.Abstractions" version="1.1.2" />
       </group>
       <group targetFramework="netstandard2.0">
-        <dependency id="NETStandard.Library" version="2.0.0" />
         <dependency id="Polly" version="6.0.1" />
         <dependency id="Microsoft.Extensions.Caching.Abstractions" version="2.0.2" />
       </group>