skip bower / bower_components during 'rails assets:precompile'

a.rb

    config = Rails.application.config

    default_precompile = config.assets.precompile

    config.assets.precompile = [
      proc do |path|
        if path =~ /bower_components/
          false
        else
          default_precompile.call(path)
        end
      end
    ]