Allow Google App Engine authentication

[razvanphp]

This is similar with PR #40

Documentation of https://github.com/Superbalist/flysystem-google-cloud-storage says:

/**
 * The credentials will be auto-loaded by the Google Cloud Client.
 *
 * 1. The client will first look at the GOOGLE_APPLICATION_CREDENTIALS env var.
 *    You can use ```putenv('GOOGLE_APPLICATION_CREDENTIALS=/path/to/service-account.json');``` to set the location of your credentials file.
 *
 * 2. The client will look for the credentials file at the following paths:
 * - windows: %APPDATA%/gcloud/application_default_credentials.json
 * - others: $HOME/.config/gcloud/application_default_credentials.json
 *
 * If running in Google App Engine, the built-in service account associated with the application will be used.
 * If running in Google Compute Engine, the built-in service account associated with the virtual machine instance will be used.
 */

but in the code keyFilePath is required.

I will prepare a PR for this if nobody beats me to it.

[schmunk42]

I will prepare a PR for this if nobody beats me to it.

PRs are always welcome ;)