Upgrade Ruby parser used

When running the representer, it outputs the following to `stderr`:

```shell
warning: parser/current is loading parser/ruby25, which recognizes
warning: 2.5.8-compliant syntax, but you are running 2.5.9.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
```

This sounds like it is using an old Ruby version. It would be great to have this use the latest Ruby parser, as otherwise newer Ruby features will likely cause the representer to break.