Fix a few more HttpAbstractions references.

samples/SelfHostServer/Startup.cs

@@ -16,7 +16,7 @@
 // permissions and limitations under the License.
 
 using Microsoft.AspNet;
-using Microsoft.AspNet.Abstractions;
+using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Server.WebListener;
 using Microsoft.Net.Server;
 

src/Microsoft.AspNet.Server.WebListener/ServerFactory.cs

@@ -38,7 +38,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Threading.Tasks;
-using Microsoft.AspNet.Abstractions;
+using Microsoft.AspNet.Http;
 using Microsoft.AspNet.ConfigurationModel;
 using Microsoft.AspNet.Hosting.Server;
 using Microsoft.AspNet.Logging;

src/Microsoft.AspNet.Server.WebListener/ServerInformation.cs

@@ -16,7 +16,7 @@
 // permissions and limitations under the License.
 
 using System.Reflection;
-using Microsoft.AspNet.Abstractions;
+using Microsoft.AspNet.Http;
 
 namespace Microsoft.AspNet.Server.WebListener
 {