Add net462 target to FsCheck.Xunit.

This should help xunit choose the right testrunner on .NET Framework.
Fixes #678.

Author: kurtschelfthout

src/FsCheck.Xunit/FsCheck.Xunit.fsproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
         <AssemblyName>FsCheck.Xunit</AssemblyName>
-        <TargetFramework>netstandard2.0</TargetFramework>
+        <TargetFrameworks>netstandard2.0;net462</TargetFrameworks>
         <GenerateDocumentationFile>true</GenerateDocumentationFile>
         <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
         <PackageDescription>