app.rb

get %r|^/github(/.+\.js)$| do
  require 'net/https'

  uri = params[:captures].first
  js = ""

  github = Net::HTTP.new("raw.github.com", 443)
  github.use_ssl = true    # do not forget this to access with HTTPS protocol
  github.start do
    ## XXX: verify the http response status
    js = github.get(uri).body
  end

  response["Content-Type"] = "application/javascript"
  js
end