Check if NSURLSessionTask is defined

When I use
```
    AFMotion::Client.build_shared(base_url) do
      header "Accept", "application/json"

      response_serializer :json
    end
```

and run

 `rake target=6.0

I'll get an error saying NSUrlSessionTask isn't there. I believe the check should be as stated in the pull request.

` http_result.rb:6:in `initialize:': uninitialized constant AFMotion::HTTPResult::NSURLSessionTask (NameError)
	from operation.rb:26:in `block in failure_block:'