Testing cache for Windows

* Hope to make Windows build faster
* https://github.com/kangwonlee/nmisp/issues/42
* As of 71d687f, without cache, each Windows test takes about 10 minutes
* 3105b4f is trying to cache `miniconda3` installation (about [14 minutes](https://travis-ci.org/kangwonlee/tutorial/builds/554683072))
* With cache, 10m47s (total cache size maybe too big?)
* For ed8160a, building cache for Windows took more than [17 minutes](https://travis-ci.org/kangwonlee/tutorial/builds/554695982)

Start by reading the linked nmisp issue and comparing the Windows CI changes in commits 71d687f, 3105b4f, and ed8160a. Check the reported cache build and test timings, then define completion as a Windows cache that reduces build time without making the total cache too large.

Vom Indexierungsmodell aus dem Issue-Text verfasst.