Test against several GDAL versions

[astrojuanlu]

CircleCI allows using custom Docker images, which would be extremely useful to test against different versions of GDAL:

https://circleci.com/docs/2.0/custom-images/#overview

Also, the Workflows functionality is more similar to what can be found in GitLab CI:

https://circleci.com/docs/2.0/workflows/

[astrojuanlu]

I amend myself: Travis CI also supports Docker https://docs.travis-ci.com/user/docker/

[astrojuanlu]

We're already testing against several Python versions