updates UPGRADING to reflect guzzle 6 changes
updates UPGRADING to reflect guzzle 6 changes

Jyo1/google_api_php,michabbb-backup/google-api-php-client,bshaffer/google-api-php-client,eRadical/google-api-php-client,googleapis/google-api-php-client,zahoorkhan/google-api-php-client,michabbb-backup/google-api-php-client