Unfreeze rake version

Change-Id: Ie19ffd85be82b27caa4fd149a3b13fec27f4f518
Reviewed-on: http://review.couchbase.org/24479
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>