Updated Watchr to Observr due to depreciation warning

Hi there,

I setting up the koans for a friend when I came across this warning when trying to use watchr.

```shell
Use RbConfig instead of obsolete and deprecated Config.
```

I found a reference to it [here](https://github.com/mynyml/watchr/pull/53), and there is a [forked project](https://github.com/kevinburke/observr) that has made this change.

So I updated the README to reflect the new project.

Thanks!