Allow download script to use wget or curl

Change-Id: I27e243d0fbaa16dac75e85d139e1c420b4686438
Reviewed-on: http://review.couchbase.org/44531
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>