introduce DefaultBranch property

Octokit.Tests.Integration/Clients/RepositoriesClientTests.cs

@@ -35,6 +35,7 @@ public class RepositoriesClientTests
                 Assert.True(repository.HasIssues);
                 Assert.True(repository.HasWiki);
                 Assert.Null(repository.Homepage);
+                Assert.NotNull(repository.DefaultBranch);
             }
             finally
             {

Octokit/Models/Response/Repository.cs

@@ -25,7 +25,7 @@ namespace Octokit
         public bool Fork { get; set; }
         public int ForksCount { get; set; }
         public int WatchersCount { get; set; }
-        public string MasterBranch { get; set; }
+        public string DefaultBranch { get; set; }
         public int OpenIssuesCount { get; set; }
         public DateTimeOffset? PushedAt { get; set; }
         public DateTimeOffset CreatedAt { get; set; }