Update underscore to latest

[pbcomm]

Would be great if we can update the underscore to latest version

[aflatter]

Hello @pbcomm,

Actually I'm thinking about removing underscore from this gem and adding a link to https://github.com/rweng/underscore-rails instead. Another idea might be to deprecate this gem altogether and point people towards rails-assets.org.

[pbcomm]

I think removing underscore would be step one. Not everyone is ok with using rails-assets yet, so It might be a good idea to keep it around for a bit longer.