Hey there,
CircleCi is pretty awesome and we/I use it for multiple D projects.
As D is a sweet language, they follow a common specification which for the majority is simply running dub test.
There is an automated, maintained installer that will automatically download the latest stable DMD compiler and DUB package manager (-> http://dlang.org/download.html).
As you can see the build helper for integrated Travis support is trivial:
https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/script/d.rb
What steps need to be taken, s.t. CircleCi can infer such a setup automatically for D projects?
I am happy to help!