Updating dependencies to work with serverless-offline

[tcbyrd]

The serverless-webpack plugin removed their own 'serve' method in v3. They recommended integrating with serverless-offline, so this PR will update everything to work locally with the new method.

Changes

• Changes the alias for npm start from sls webpack serve to sls offline run
• Changes index.js to handler.js
  • Adds an associated output object to the webpack config to make sure it bundles everything into /.webpack/handler.js. In the new version of serverless-webpack, it was defaulting to main.js, so since I had to override it anyway, I decided to make it hander.js to be consistent with other examples.
  • Updates the handler event in serverless.yml
• To make it possible for a single project to contain multiple probot plugins with different handlers, this now exports the example plugin as autoResponder.

I'll deploy this to Lambda and update tcbyrd-probot to verify before merging.

[tcbyrd]

🌮 🎉 Verified it still works after this update! 🎉 🌮