Refactor asynchronous calls in the code to use async/await

[diegodorgam]

So now we are using Node8, we can make use of async instead of using chains of promises. We have two files to refactor:

• https://github.com/RocketChat/hubot-natural/blob/252576dc86a328e22250a5f19c75c3c47e6ef66d/scripts/actions/respond.coffee#L26
• https://github.com/RocketChat/hubot-natural/blob/b16e3b6068fb1b05625b52d8f778b9cfa17d5082/scripts/lib/security.coffee#L5

@ArthurTemporim are you up to do this?