Fix Retina screen detection in Ruby specs

Author: p0deje

rb/spec/integration/selenium/webdriver/spec_support/helpers.rb

@@ -98,7 +98,7 @@ def png_size(path)
           width = png.unpack1('NN')
           height = png.unpack('NN').last
 
-          if Platform.macos? # Retina
+          if Platform.mac? # Retina
             width /= 2
             height /= 2
           end