slurm: 17.11.5 -> 17.11.7, pyslurm: 20180427 -> 20180604 (#42536)

* slurm: 17.11.5 -> 17.11.7, pyslurm: 20180427 -> 20180604

This commit also swaps to use the official repository's github release tags
instead of their download site, which only keeps the most recent version with no
historical archives.

* Document why we don't run tests

* Remove dead testing code