Red Tests were fixed

Unused 'using' directive were removed.

Octokit.Tests/Reactive/ObservableTreesClientTests.cs

@@ -1,16 +1,23 @@
 using System;
-using System.Reactive.Linq;
 using System.Reactive.Threading.Tasks;
 using System.Threading.Tasks;
 using NSubstitute;
 using Octokit.Reactive;
-using Octokit.Tests.Helpers;
 using Xunit;
 
 namespace Octokit.Tests
 {
     public class ObservableTreesClientTests
     {
+        public class TheCtor
+        {
+            [Fact]
+            public void EnsuresArgumentIsNotNull()
+            {
+                Assert.Throws<ArgumentNullException>(() => new ObservableTreesClient(null));
+            }
+        }
+
         public class TheGetMethod
         {
             [Fact]