Update NLog dependency to 4.6.8 (#408)

Author: 304NotModified

src/NLog.Extensions.Logging/NLog.Extensions.Logging.csproj

@@ -66,7 +66,7 @@ Full changelog: https://github.com/NLog/NLog.Extensions.Logging/blob/master/CHAN
     <DefineConstants>$(DefineConstants);NETSTANDARD</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="NLog" Version="[4.6.7,5.0.0-beta01)" />
+    <PackageReference Include="NLog" Version="[4.6.8,5.0.0-beta01)" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'net451' ">
     <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="1.0.0" />