app.rb

# create a route that will respond to an incoming POST request from Twilio
post '/message' do
  # Create a TwiML response object - this object helps us generate an XML string
  # that we will return as the result of this request
  twiml = Twilio::TwiML::Response.new do |r|
    r.Message do |m|
      m.Body 'Trust Pound!'
      m.Media 'http://i.imgur.com/Act0Q.gif'
    end
  end

  # set the Content-Type header of the response for XML
  content_type 'text/xml'
  twiml.text
end