State System.ValueTuple dependency for Net Standard 1.1 target

Author: reisenberger

src/Polly.Caching.Serialization.Json.nuspec

@@ -34,6 +34,7 @@
       <group targetFramework="netstandard1.1">
         <dependency id="NETStandard.Library" version="1.6.1" />
         <dependency id="Newtonsoft.Json" version="11.0.2" />
+        <dependency id="System.ValueTuple" version="4.5.0" />
         <dependency id="Polly" version="7.0.0" />
       </group>
       <group targetFramework="netstandard2.0">