Add Rails 6.0 as a Runtime dependecy or some kind of warning

[maxkerp]

Hi there, just a little feedback.

We are still on rails 5.2 for one of our apps and did not update the gem for a while. Now that we did we ran into an issue with the method ActionView::Template::Handlers::ERB.call which does not have the source parameter on rails 5.2.

So if I don't miss something this gem is only compatible with rails ~> 6.0 starting with version 0.3.5.

Adding this to your runtime dependency or some kind of warning or even an Exception when its used with rails <= 6.0 could be really helpful and save the next developer some time.

Anyway thanks a lot for providing this gem! It's much appreciated ❤️