ci: Switch CI configuration to boostorg/boost-ci

Travis CI is dead. Our CI has rotten. We should consider optimising and eliminate inefficient, repeated builds, improve CI maintenance.

The https://github.com/boostorg/boost-ci offers Boost-standardized configuration for all major CI services.
It is compelling to switch our home-grown configuration to it. So, let's discuss it.

### Major issues with boost-ci that we need to address:

- We need to disable certain compilers (e.g. GCC <5) and we need to comment entries in YAML files - we have to modify the boost-ci files anyway
- We need to build multiple targets e.g. separate core from I/O, or run I/O tests iff core tests pass, but boost-ci always builds `b2 libs//test`.
- We need to install third-party dependencies for I/O (using `apt`, `conan` or `vcpkg`)
- ...

### Playground

- https://github.com/mloskot/gil/tree/ci branch with boost-ci for GIL prototype
- https://github.com/mloskot/gil/pull/3 as playground