Travis builds no longer running

Visting https://app.travis-ci.com/github/con/tinuous shows that no builds have run since October 2022, and (if you're logged in?) there is a banner with the message:

> Builds have been temporarily disabled for public repositories due to a negative credit balance. Please go to the [Plan page](https://app.travis-ci.com/organizations/con/plan) to replenish your credit balance or alter your [Consume paid credits for OSS setting](https://app.travis-ci.com/organizations/con/plan?anchor=oss-consumption).

We can either pay for more Travis builds or stop testing this repository with Travis. The latter would mean that we would need some other Travis repository to run `tinuous-inception` or similar on in order to test `tinuous`'s Travis support.