refactor!: update code and remove underscore dependency

[dhensby]

This change updates the code in the following ways:

1. Remove var usage and replace with let/const where appropriate
2. Removes the dependency on underscore and replaces the helper methods with native code
3. package.json is slightly refactored to set the entry point as src/server.js instead of index.js - as this isn't really needed

I'd consider this potentially to be a breaking change, though I was surprised to see it still maintains Node 8+ support.