Fixed wrong target framework in DataAnnotations.Tests project (#278)

Author: ivaylokenov

test/MyTested.AspNetCore.Mvc.DataAnnotations.Test/MyTested.AspNetCore.Mvc.DataAnnotations.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.0;net461</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.1;net461</TargetFrameworks>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <AssemblyName>MyTested.AspNetCore.Mvc.Test</AssemblyName>
     <AssemblyOriginatorKeyFile>../../tools/Key.snk</AssemblyOriginatorKeyFile>