Set date in RELEASE_NOTES

Change-Id: I5fce32f97913a3e2e073be4b367eb0d0114ecc2b
Reviewed-on: http://review.couchbase.org/27352
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>