Update to newest fcm API endpoints

[matzew]

Done to match latest Server endpoint changes, discussed in https://developers.google.com/cloud-messaging/android/android-migrate-fcm#update_server_endpoints

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[matzew]

@googlebot I signed it !

Well, not directly, but I work for Red Hat, and there is a CLA for Red Hat / Google in place. My understanding is that not just the Kubernetes work is covered by that CLA

[matzew]

@googlebot ping

[cbornet]

Wouldn't it be better to be able to pass a user defined url to the Sender() constructor ?

[cbornet]

See #168. FCM could be made the default endpoint though