Add --no-pngout flag for image_optim command on :fail message

Because `pngout` is skipped in `ImageOptim.new(:pngout => false)`, it
should be skipped too or else it will fail silently when `pngout` is not
installed.

Change-Id: I9da246fef88b5215b403dbf3fa649ad24647b39c
Reviewed-on: http://gerrit.causes.com/43318
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>