added build of all-images.js in only-web.rb

Change-Id: I87b8f319728141c27d7b3dca8d80fc4d2138f3bf
[alk@tut.by: changed to more idiomatic ruby]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/28595
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>