Fix #75: Fix Travis CI deployments from branches other than master

[MaximeKjaer]

The previous deployment failed with the following message:

Skipping a deployment with the releases provider because this branch is not permitted: 0.4.2

It turns out that the previous config only allowed deployments from master. According to the Travis CI docs:

branch: name of the branch. If omitted, this defaults to the app-specific branch, or master. If the branch name is not known ahead of time, you can specify all_branches: true instead of branch: and use other conditions to control your deployment.

In our case, we already only deploy if a tag is present, so all_branches: true should be fine.

Hopefully it'll work this time 🤞