Bugfix/1.1/#664 release publish optipng

pull request for develop.

optipng was applied to intermediate product for smart spritesheet creation which lead to file not found errors. harmless but not helpful for users.

fixes: https://github.com/spritebuilder/SpriteBuilder/issues/664

forum: http://forum.spritebuilder.com/t/errors-publishing-as-release-issue-raised/1296