Fixed double Downloader() next event on redirect.
Fixed double Downloader() next event on redirect.

The runNextUri() function should not be called when the next event fires
(which happens on a redirect) because runNextUri() emits the next event
internally. This was causing a new concurrent download to happen on
every redirect (which happens on every image). Related to pull request
#3.

stevenbenner/interfacelift-downloader,ashking/interfacelift-downloader,sunpech/interfacelift-downloader