Apparently, this was checking the wrong path in Travis CI, and thus
never actually running (meaning we'd accumulate every `node_modules`
directory ever in the Travis caches, which in turn resulted in very
slow builds).
This commit re-arranges the arguments of `purge_unused_caches()`
function in order to remain consistent with other similar functions
in the library like `may_be_perform_caching()`.
Based on the `dry_run` flag, this function either purges the list
of directories passed to them or prints a listing of the directories
it would have purged/kept_back, had the `dry_run` flag been false.
Tim Abbott
Harshit Bansal