Remove "Rails 4" misleading sentences from readme

[collimarco]

This sentence is confusing:

The final installation step is dependent on your version of Rails. If you're not running Rails 4, skip to the next section. Otherwise, create the following migration:

I am using Rails 7, so I did not create the migration... And I got errors.

I recommend to remove that sentence and all the parts about Rails 4 from readme.

Nobody is using Rails 4 now and in any case it would be a terrible security practice to use old unsupported versions.