Update the dependency group for .net standard 2.0 to use the abstractions caching package rather than memory.

Author: SeanFarrow

src/Polly.Caching.Distributed.nuspec

@@ -36,7 +36,7 @@
       <group targetFramework="netstandard2.0">
         <dependency id="NETStandard.Library" version="2.0.0" />
         <dependency id="Polly" version="6.0.1" />
-        <dependency id="Microsoft.Extensions.Caching.Memory" version="2.0.2" />
+        <dependency id="Microsoft.Extensions.Caching.Abstractions" version="2.0.2" />
       </group>
     </dependencies>
   </metadata>