Audit devDependencies

[pdehaan]

Steps to reproduce:

$ git rev-parse --short HEAD # 0e42eee
$ npx depcheck --ignores=eslint-*,karma-*

Unused devDependencies
* babel-core
* babel-preset-env
* chai # NOTE: Possibly used in karma.conf.js
* fs-extra
* fx-runner
* get-firefox # NOTE: Possibly used in .circleci/config.yml
* jshint
* minimist
* sinon # NOTE: Possibly used in karma.conf.js
* sinon-chrome  # NOTE: Possibly used in karma.conf.js

Missing dependencies
* semver # NOTE: Possibly used in test/ensure_minimum_node_version.js (but not in package.json)

I think we're using chai, sinon, and sinon-chrome is used in karma.conf.js, and get-firefox is used by Circle-CI. I think the rest are possibly unused.

[motin]

@pdehaan Could you update this issue based on the proposed PR #73? Thanks