Update ImageOptim dependency to 0.18.0

`ImageOptim` now supports a `skip_missing_workers` option, which allows
us to not have to manually disable `pngout`.

Change the hook to use this option and update the dependency.

Change-Id: Iea175e9139b7f8e64d75a1da806d70241e618d37
Reviewed-on: http://gerrit.causes.com/45274
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>