Yep, just call as you would and use the simple API access key - you can retrieve public information, but you will have to supply to long numeric ID for the user you're retrieving posts for rather than using the string 'me'. Take a look at the latest version of the library as well: https://github.com/google/google-api-php-client as well. You need to setup your client as you were doing, with an API key from a project which has the Google+ enabled on https://developers.google.com/console
$client = new Google_Client();
$client->setApplicationName("Post Fetcher");
$client->setDeveloperKey('PUT_YOUR_API_KEY_HERE_OR_IT_WONT_WORK');
$plus = new Google_Service_Plus($client);
$activities = $this->plus->activities
->listActivities("118051310819094153327", "public");