Update Firebase url's in README.md

[ghost]

Firebase moved from www.firebase.com to firebase.google.com.

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[ghost]

I signed it!

[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 93.172% when pulling f6100d7 on birkskyum:patch-1 into 0f0cc2a on firebase:master.

[jwngr]

Thanks for the PR, but this and much more is already being handled in #717. That should get merged in soon.