Add netstandard2.0 target

Ix.NET/Source/System.Linq.Async/System.Linq.Async.csproj

@@ -1,7 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net46;netstandard1.4</TargetFrameworks>
+    <TargetFrameworks>net46;netstandard1.4;netstandard2.0</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup>
@@ -37,8 +37,4 @@
     </None>
   </ItemGroup>
 
-  <ItemGroup>
-    <Service Include="{508349b6-6b84-4df5-91f0-309beebad82d}" />
-  </ItemGroup>
-
 </Project>