Add Node 8, drop Node 4

[honzajavorek]

🚀 Why this change?

• Node 8 is the latest even production-ready version and we should be sure Dredd supports it correctly.
• Node 4 end of life is April 2018 and there is no reason in supporting it anymore.
• We should not rely on loose dependency versioning - because there's no guarantee the interface, license, etc. won't change in minor or patch version. Semantic versioning is a nice idea, but not everyone follows it. Let's turn on Greenkeeper for this repo and let's pin all dependencies. Also adding package-lock.json as that's the standard way to work with npm@5 now.

📝 Related issues and Pull Requests

• Closes Support for Node 8 when it becomes LTS #754
• When using Node 8, the console is available for the sandboxed code pitboss#13

✅ What didn't I forget?

• To write docs
• To write tests
• To put Conventional Changelog prefixes in front of all my commits and run npm run lint

[honzajavorek]

@michalholasek If you like the changes, leaving up to you whether you want to merge this now or cherry-pick the commits at the end of your decaff PR and merge those together when the thing is ready.

[michalholasek]

@honzajavorek I am ok with merging it before decaff branch is done, just one note - shouldn't we consider this a breaking change, since we are dropping Node 4 support?

[honzajavorek]

@michalholasek It is a breaking change: