To support this, the following changes have to be made as well:
- Ignore `package-lock.json` since NPM creates it, but we should not
have it in the repository.
- Switch from `babel-preset-es2015` to `babel-preset-env` to resolve
the deprecation warning in the test logs. The latter is more recent
and flexible, but should be the same functionality-wise.
- `transform` now needs to have the `utf-8` encoding option provided.
If not given, it will call the callback with a `Buffer` object,
which results in an unhandled promise rejection since what is
returned from the callback is a string, not a `Buffer`.
