Silence image_optim warnings in development

Lots of new developers see this during configuration (e.g. rails
db:migrate) and then spend time trying to fix them. So let's avoid
the warnings entirely.

config/image_optim/development.yml

@@ -0,0 +1,12 @@
+skip_missing_workers: true
+advpng: false
+gifsicle: false
+jhead: false
+jpegoptim: false
+jpegtran: false
+optipng: false
+oxipng: false
+pngquant: false
+pngout: false
+pngcrush: false
+svgo: false