Results 4 issues of pozorvlak

See conversation on #1069.

![growstuff_ram](https://user-images.githubusercontent.com/225991/39879830-c8c221c4-5473-11e8-9d45-b0a1e84d6bd1.PNG) Here's [Heroku's guide to optimizing memory usage for Ruby](https://devcenter.heroku.com/articles/ruby-memory-use).

type: technical improvement
bug - critical

... is AFAICT fine, but can be simplified. Suppose X' != X. Then there exists some x which is in X' but not in X. Since {X,Y} is a partition...

Currently they're put in /tmp, and then thrown away when the container is torn down at the end of the build. There are a few options for uploading them to...

type: technical improvement
effort: small
needs: access rights
type: travis ci