Update lib/smart_asset.rb

This option errors out:

TypeError: Cannot call method 'walk' of undefined
    at getOptions (/usr/local/share/npm/lib/node_modules/uglify-js/bin/uglifyjs:325:13)
    at Object.<anonymous> (/usr/local/share/npm/lib/node_modules/uglify-js/bin/uglifyjs:109:16)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)