We use the globally installed yarn now. The local yarn was used in
`check-environment.js` only, which results in the `--integrity` check
always failing (if dependencies were installed with the global yarn).
Travis creates an empty node_modules directory when the cache is empty which confuses
our current script into thinking that it's ok to require fs-extra. While this is rare,
it's better not to depend on anything in node_modules when purging it, so I reimplemented
recorsive delete that we use to purse node_modules.
George Kalpakas
Olivier Combe
Victor Berchet
Chuck Jazdzewski
Pawel Kozlowski
Brian Ford
Igor Minar