CI: Improve CircleCI
Not sure how much of that needs to be done upstream, but right now, CircleCI is performing very poorly. 

We're on the sixth page of jobs (so about 24h waiting time for commits coming in now), and CircleCI is stalling intermittently, e.g. right now, since about half an hour (not sure if those workers may be changing to some private pipelines that are not visible to me):

![4e](https://user-images.githubusercontent.com/33685575/45756386-0a723680-bc21-11e8-9842-6bfd19939bae.png)

Further improvements that would speed things up:
* skip jobs also if there are newer force-pushed (e.g. squashed) commits
* respect `[ci skip]` if issued
* sometimes, only 1-2 workers are running (on change-over between PRs), instead of 4
* more workers?
* etc...
