Set defaultLocale and locale values from Rails' I18n module

Move the rather awkward looking
```javascript
I18n.defaultLocale = "<%= I18n.default_locale %>";
I18n.locale = "<%= I18n.locale %>";
```
configuration [mentioned in the readme](https://github.com/fnando/i18n-js#on-the-javascript) into the gem itself. It is rather unnecessary to require this configuration, since most use cases probably will configure this the exact same way - and it can still be overwritten.