OS X Yosemite fixes

When using your library in conjunction with [Octokit](https://github.com/octokit/octokit.objc), I noticed that you code references several constants marked as deprecated starting in OS X Yosemite. I replaced references with them to their recommended replacements.

Note that I couldn’t verify that the library would still compile under the Mavericks SDK with these changes. You may want to check that before you merge.