Specify OS X platform support in Podspec

This adds OS X support to the Podspec. Just FYI, running lint on the Podspec found a couple of other issues:

```sh
$ pod spec lint Airbrake-iOS.podspec

 -> Airbrake-iOS (4.1.2)
    - WARN  | The URL (http://airbrake.io/pages/ios-notifier) is not reachable.
    - ERROR | [xcodebuild]  Airbrake-iOS/Airbrake/notifier/ABCrashReport.h:10:9: fatal error: 'CrashReporter/CrashReporter.h' file not found

Analyzed 1 podspec.

[!] The spec did not pass validation.
```