Fix wrong version in nuget

Build/MQTTnet.nuspec

@@ -18,7 +18,7 @@
     <dependencies>
 
       <group targetFramework="netstandard1.3">
-        <dependency id="NETStandard.Library" version="1.3" />
+        <dependency id="NETStandard.Library" version="1.6.1" />
         <dependency id="Microsoft.Extensions.DependencyInjection" version="1.1.1" />
         <dependency id="Microsoft.Extensions.Logging" version="1.1.2" />
         <dependency id="Microsoft.Extensions.Options" version="1.1.2" />