It was not possible, but with the help of Travis the introduction of their new container infrastructure, now they publish the IP ranges of their machines. You can find them here:
https://docs.travis-ci.com/user/ip-addresses/
For Travis Pro you need to take the IP range specified for travis-ci.com ; open source projects need travis-ci.org projects. There are no fixed ranges of IP addresses yet if you are using outdated infrastructure . If your project does not require sudo , take a look at the migration guide .