I agree with the answer provided by @Leeft. In case of a situation when you need only one file, you can refer to the Raw file specified in github in your installation.
bower install https://raw.githubusercontent.com/chrishunt/retinajs/master/src/retina.js --save
This will include the dependency in your bower.json file
"dependencies": { "retina": "https://raw.githubusercontent.com/chrishunt/retinajs/master/src/retina.js" }
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