Add support for Node v8 in aws lambda

[inian]

Changing the environment to node v8 and deploying doesn't work.

[tomardern]

I recommend that the Docs are changed to state that this package only currently works for Node 6.10

[cirdes]

as mentioned on #25 to run on 8.10 we just have to update the nodejs on .babelrc("8.10") and on serverless.yml (nodejs8.10) and change the handler to:

exports.handler = async (event) => {
  let result;
  let browser;
  try {
    browser = await setup.getBrowser();
    result = await exports.run(browser);
  } catch (err) {
        console.log(err);
        return err;
  }
  return result;
};