LoggerConfiguration not referenced when using core 1.1.0

Everything is fine when in version netcoreapp1.0 but when upgrading to netcoreapp1.1 / NetCore.App 1.1.0, compilation fails with the message The type 'IEnumerable<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'

and points to : Log.Logger = new LoggerConfiguration()

here are my project.json dependencies

"dependencies": {
    "Microsoft.AspNet.WebApi.Core": "5.2.3",
    "Swashbuckle": "6.0.0-beta902",
    "Microsoft.AspNetCore.Server.Kestrel": "1.1.0",
    "Microsoft.Data.Services.Client": "5.8.2",
    "Microsoft.Extensions.Caching.Abstractions": "1.1.0",
    "Microsoft.Extensions.Caching.Memory": "1.1.0",
    "Microsoft.Extensions.Configuration.EnvironmentVariables": "1.1.0",
    "Microsoft.Extensions.Configuration.FileExtensions": "1.1.0",
    "Microsoft.Extensions.Configuration.Json": "1.1.0",
    "Microsoft.Extensions.Logging": "1.1.0",
    "Microsoft.Extensions.Logging.Console": "1.1.0",
    "Microsoft.Extensions.Logging.Debug": "1.1.0",
    "Microsoft.Extensions.Options.ConfigurationExtensions": "1.1.0",
    "Microsoft.IdentityModel.Protocols": "2.1.2",
    "System.Spatial": "5.8.2",
    "Microsoft.NETCore.App": {
        "version": "1.1.0",
        "type": "platform"
    },
    "AutoMapper.Extensions.Microsoft.DependencyInjection": "1.2.0",
    "Microsoft.AspNetCore.Mvc.WebApiCompatShim": "1.1.0",
    "Microsoft.AspNetCore.Routing": "1.1.0",
    "Microsoft.AspNetCore.Mvc": "1.1.0",
    "Microsoft.AspNetCore.Server.IISIntegration": "1.1.0",
    "Mindscape.Raygun4Net.AspNetCore": "5.3.2",
    "Serilog": "2.4.0",
    "Serilog.Sinks.Literate": "2.1.0",
    "Serilog.Extensions.Logging": "1.4.0",
    "Serilog.Sinks.RollingFile": "3.3.0",
    "Serilog.Sinks.Seq": "3.2.0",
    "Serilog.Sinks.Raygun": "2.0.13"
},

"frameworks": {
    "netcoreapp1.1": {
        "imports": [
            "dotnet5.6",
            "dnxcore50",
            "portable-net45+win8",
            "net462"
        ]
    }
},

MindscapeHQ / serilog-sinks-raygun

LoggerConfiguration not referenced when using core 1.1.0 #9