CI: Remove travis

Travis introduced OSS credits a while ago, which artificially limit the OSS development by putting a pressure on how much time we can take for a build. While I can understand this decision, considering they're providing the service for free, there is no longer any reason for us to keep using it, due to heavy maintenance burden and pressure, something we really don't need considering other CIs (especially GitHub actions) do not have such limitations.

This is on top of other issues, which make Travis one of the most problematic CIs that I've ever tried to use.

README.md

@@ -2,7 +2,6 @@
 
 [![Build status (GitHub)](https://img.shields.io/github/workflow/status/JustArchiNET/ArchiSteamFarm/ASF-ci/main?label=GitHub&logo=github&maxAge=600)](https://github.com/JustArchiNET/ArchiSteamFarm/actions?query=workflow%3AASF-ci+branch%3Amain)
 [![Build status (AppVeyor)](https://img.shields.io/appveyor/ci/JustArchi/ArchiSteamFarm/main.svg?label=AppVeyor&logo=appveyor&maxAge=600)](https://ci.appveyor.com/project/JustArchi/ArchiSteamFarm)
-[![Build status (Travis)](https://img.shields.io/travis/com/JustArchiNET/ArchiSteamFarm/main.svg?label=Travis&logo=travis&maxAge=600)](https://travis-ci.com/JustArchiNET/ArchiSteamFarm)
 [![Build status (Docker)](https://img.shields.io/github/workflow/status/JustArchiNET/ArchiSteamFarm/ASF-docker-publish-main.svg?label=Docker&logo=docker&maxAge=600)](https://hub.docker.com/r/justarchi/archisteamfarm)
 [![Github last commit date](https://img.shields.io/github/last-commit/JustArchiNET/ArchiSteamFarm.svg?label=Updated&logo=github&maxAge=600)](https://github.com/JustArchiNET/ArchiSteamFarm/commits)
 [![License](https://img.shields.io/github/license/JustArchiNET/ArchiSteamFarm.svg?label=License&logo=apache&maxAge=2592000)](https://github.com/JustArchiNET/ArchiSteamFarm/blob/main/LICENSE-2.0.txt)