Update example article for Rails 5.1

[wrycoder]

When I tried to go through the steps in the example...

http://collectiveidea.com/blog/archives/2016/05/31/beyond-yml-files-dynamic-translations/

...my test kept failing. It appeared as if the TranslationsController was being ignored. The locale was never set, but the application views still loaded. After lots of trial and error, I came across this post:

https://stackoverflow.com/questions/16519828/rails-4-before-filter-vs-before-action

It looked as if the example was using a method that has now been removed from Rails. When I changed before_filter to before_action, everything worked. Could the example article be updated?

[timfjord]

I am afraid I don't have access to the article so I won't be able to correct it.
At the same time, I would like to keep the article in the README.

What I can do is note that the article is not suitable for Rails 5.1+

[timfjord]

Here we go 1fe4b91

[wrycoder]

Thanks!