Update dependencies

[36degrees]

What

• Bump webpack-dev-server from 3.1.10 to 3.1.11
• Bump mixin-deep from 1.3.1 to 1.3.2
• Bump js-yaml from 3.12.0 to 3.13.1
• Bump handlebars from 4.0.12 to 4.7.6
• Bump acorn from 5.7.3 to 5.7.4

Why

Dependabot has raised a number of PRs against this repo:

#455
#456
#457
#458
#459

Given how long the tests take to run, and the fact that every open PR is automatically rebased when one of them is merged, it's quicker to raise a single PR that bumps all of the outdated dependencies.