Add new ctor to PublicRepositoryRequest taking "since" parameter as a long, and obsolete the old ctor that uses an int

Update tests to pass long datatype

Octokit.Tests.Integration/Reactive/ObservableRepositoriesClientTests.cs

@@ -37,7 +37,7 @@ namespace Octokit.Tests.Integration
                 var github = Helper.GetAuthenticatedClient();
 
                 var client = new ObservableRepositoriesClient(github);
-                var request = new PublicRepositoryRequest(32732250);
+                var request = new PublicRepositoryRequest(32732250L);
                 var repositories = await client.GetAllPublic(request).ToArray();
                 Assert.NotEmpty(repositories);
                 Assert.Equal(32732252, repositories[0].Id);