Travis job failing for Firebase client deployment to GCP

[allishultes]

Describe the bug
A clear and concise description of what the bug is.
The Travis job for deploying the changes to the client in this repo to GCP hosting is not working. Deployments on master are failing with a 401 error:

Related jobs:

• https://travis-ci.org/github/bbc/digital-paper-edit-firebase/builds/771014932
• https://travis-ci.org/github/bbc/digital-paper-edit-firebase/builds/771012585
• https://travis-ci.org/github/bbc/digital-paper-edit-firebase/builds/770721949

To Reproduce
Steps to reproduce the behaviour:

N/A - you can try triggering a build by merging into Master, but I'd probably just start by checking out the logs.

Expected behaviour
A clear and concise description of what you expected to happen.

The Travis job should pass when changes are merged to master, and should be reflected on the GCP-hosted site.

Additional context
Add any other context about the problem here.

You can deploy to Firebase from the terminal so this doesn't block us from updating the site, it's just one more finicky thing to remember for our rather long deployment process.

[allishultes]

Potentially useful: https://cloud.google.com/architecture/continuous-delivery-with-travis-ci