partially fix bug w/ dirs on recent macOS versions
partially fix bug w/ dirs on recent macOS versions

There's a bug on recent macOS versions where using a directory as the
wallpaper source with setwp prevents the Dock application from
restarting after being killed (see issue #3). This commit partially
fixes it, in the sense that it makes it works most of the time but not
always.

alexandrecormier/setwp,alexandrecormier/setwp