Tests fail because chalk.bold does not exist (on my local machine)

* What version of documentation.js are you using?:
6.3.0
* How are you running documentation.js (on the CLI, Node.js API, Grunt, other?):
on the cli, `npm run test`, ubuntu 17.10, node 8.11.1

The output complaints about the chalk module. Current version of chalk is 2.3, 2.4 also fails:
```

flow files 51
non flow files 6
------------------
src/default_theme/assets/anchor.js
src/default_theme/assets/site.js
src/default_theme/assets/split.js
src/default_theme/index.js
src/hierarchy.js
src/index.js
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ src/commands/readme.js:106:17

Cannot call chalk.bold because property bold is missing in module chalk [1].

[1] 9│ const chalk = require('chalk');
:
103│
104│ if (argv.d) {
105│ log(
106│ chalk.bold(`${argv.readmeFile} needs the following updates:`),
107│ `\n${diffOutput}`
108│ );
109│ process.exit(1);

```

I am aware this is due to my local environment, but I have no clue what part of it. Any pointers would be appreciated.