GithubUtils: fileContents throwing error

Using the following `dangerfile.js` (ignore hard coded values ☺️):

```js
"use strict";

schedule(async function() {
const fileContents = await danger
.git
.created_files
.map(p => danger.github.utils.fileContents(p, "danger/danger-js", "relative_files"))

console.log(fileContents);
})
```

And running the command:

```sh
$ yarn danger pr https://github.com/danger/danger-js/pull/359
```

I'm seeing the following error:

```
TypeError: this.getFileContents is not a function
```

-------------

Been trying to debug but can't quite find the why this would be failing: https://github.com/danger/danger-js/blob/b6348da0eab7935ccbc2c68082ed558fa052c4dc/source/platforms/github/GitHubAPI.ts#L72-L74

I hope it's not my local env (installed it via yarn) and should be easily reproducible with the above.