added parentheses, ruby was complaining without them there

Was getting errors:

/Library/Ruby/Gems/1.8/gems/compass-retina-1.0.0/lib/compass/sass_extensions/functions/retina_images.rb:4: warning: don't put space before argument parentheses

and 

TypeError on line 5 of /Library/Ruby/Gems/1.8/gems/compass-retina-1.0.0/lib/compass/sass_extensions/functions/retina_images.rb: can't convert Array into String

Adding parentheses around the arguments of File.join fixes this