add rubysl gem if RUBY_ENGINE is rbx

after the app creation when I try to launch rails command
the following exception is raised using Rubinius 2:

An exception occurred running bin/rails:

     (LoadError)
    The library "delegate" is a provided by the "rubysl-delegate" gem and needs to be
    installed to be loaded.

    If using Bundler, add this library or the standard library meta-gem to the
    Gemfile as follows:

      gem "rubysl-delegate", "~> 2.0"

        OR

      gem "rubysl", "~> 2.0"

....