Restore Android and iOS builds on another CI service

[mbrukman]

There's still a .travis.yml symlink and a travis.yml config in the repository, but looks like Travis CI is no longer being used for this project:

• https://travis-ci.org/github/mosra/magnum says:

  This repository was migrated and is now building on travis-ci.com

• the most recent build on https://travis-ci.com/github/mosra/magnum is from 7 months ago

Looks like this project is currently using AppVeyor and Circle CI instead, based on the other badges in the README.

Would it make sense to remove these files, and the Travis CI badge? Or if it's still in use, perhaps it makes sense to reactive the builds?

[mosra]

Hi!

The issue is, that Travis said dot org will get shut down, gradually making it less and less usable, forcing me to migrate to dot com. But shortly after they cut off everyone from the service, demanding to either pay about $70/month or beg them for "free OSS credits" with extremely restrictive rules. I sent them several e-mails trying to negotiate a solution but they just ghosted me and nothing happened in the last 7 months. So I slowly migrated what I could to CircleCI.

What's left is an iOS and Android build that made use of device emulator on Travis CI to run unit tests. Those are nontrivial to port elsewhere (probably would need to use some paid service to test on real devices), and because I didn't find time to look into that yet, I left the relevant files and configs in the repo. This is a blocker for the next release, so I have to restore builds for these platforms one way or another.

[mosra]

Remaining Android builds got migrated in eaa95dd (now also including tests for GLES and Vulkan functionality on SwiftShader), iOS builds in d3a411c, the now-obsolete Travis config files are removed as of 89e3c52. Since a7b94a4 there's also a new ARM64 build because both the Android and the iOS build are for an x86 {e,si}mulator now and thus there wouldn't be any build testing an ARM target otherwise.

Similar change is being done in other Magnum repositories.