[WIP] Rails 6 support

[prathamesh-sonpatki]

No description provided.

[schneems]

I ported over your code and got it across the finish line here #50

Thanks a ton for your work! The missing pieces were that when creating the new template object we have to pass in source, and strangely enough the html templates' format is now nil instead of :html. There might be a bug in our code somewhere causing that but it was easier to flip the logic and check for not :text.