CI: migrate to new CircleCI format

[jethron]

It's a lot bigger than the old format, sorry!

I'm not sure what you care about, CI-wise, for this library, so I went with the Debian release branches so we can at least see compilation isn't going to break as the Linux ecosystem evolves. Easy enough to change if that's too much or too little.

Switched to the nodejs coveralls project because the Ruby one doesn't seem to support Circle, I'm assuming that COVERALLS_REPO_TOKEN is already setup there and will hopefully just start working again, didn't test this myself.

The other dependencies seem to all be included in circleci/buildpack-deps, unless you want to add the lua support or other build options.

Looked at adding test tracking as well, but it will involve modifying the tests to support TAP output to convert to xunit or cucumber for Circle to support that. It can be a different PR if that's valuable.

Also wasn't sure how to run the tests with cmake?

Let me know if you'd like any changes, thanks for your work on UCL!

[bapt]

to be honnest if the CI was to be moved I would clearly prefer cirrus-ci just for the fact that is supports way more OS then just linux. For instance it can fun on FreeBSD

[vstakhov]

We have drone ci running for Rspamd. I'm fine with any CI option TBH.