To create favicon.ico, you don’t need anything other than ImageMagick.

brew install imagemagick
sudo apt-get install imagemagick

Use it to convert PNG’s into .ico: (use 32px and 16px sizes for retina compatibility)

convert favicon-32.png favicon-16.png favicon.ico

You can even use it to generate a 16px version from a 32px:

convert favicon-32.png -resize 16x16 favicon16.png