Update changelog

[hypesystem]

The changelog has fallen behind as of recent. We should update it to make sure it reflects which changes were introduced in which versions.

[eladnava]

Seems like this is partially addressed via https://github.com/ToothlessGear/node-gcm/pull/319/files#diff-4ac32a78649ca5bdd8e0ba38b7006a1eR4.

[hypesystem]

Yes! That's what made me realize we had forgotten to do this for a while. But we need a bit more detail, and to make sure that we include all the versions we have missed.

[Schweinepriester]

Updating the changelog would be helpful :)

[mdesson]

Are there any known breaking changes within v1.x.x? I know this project uses semver, but I usually read through the changelog of a given package to validate before going through an upgrade. An updated changelog would be a great help