Clean the repo before storing circleci artifacts

[eteq]

This comes from #97 and some of the related discussion in #99. Specifically, https://github.com/spacetelescope/notebooks/issues/97#issue-442833535 is relevant here - now that #99 is merged we may want to do what @obviousrebel suggests there of cleaning before storing the artifacts.

My primary concern with that is mainly https://github.com/spacetelescope/notebooks/pull/99#pullrequestreview-245622676 - that is, that maybe we want those artifacts to help debug when something goes wrong. But maybe I'm worrying too much? Or we could do this provisionally but if we run into issues later we can roll it back?

cc @laurenmarietta (and @obviousrebel if you have any residual opinion on this)

[astro-josh]

I do see the value of keeping all artifacts, they were helpful in finding an issue found in PR#77 but they do add significant time to the build.

Perhaps creating an environment variable called CLEAN in circleCI that would determine if the repo should be cleaned or not cleaned prior to storing artifacts? That way, if the need arises to debug or view artifacts this environment variable could easily be switched to not clean and store all artifacts to help view useful logs and such.