fix(csproj): target netstandard instead of core

Author: jaredcnance

src/JsonApiDotNetCore/JsonApiDotNetCore.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <VersionPrefix>2.0.0</VersionPrefix>
-    <TargetFramework>netcoreapp1.0</TargetFramework>
+    <TargetFrameworks>netstandard1.6</TargetFrameworks>
     <AssemblyName>JsonApiDotNetCore</AssemblyName>
     <PackageId>JsonApiDotNetCore</PackageId>
     <RuntimeFrameworkVersion>1.1.1</RuntimeFrameworkVersion>