Fix Vanity::Rails::Helpers#vanity_js for Rails 6; and fix Appraisal definitions

[ariel-codes]

Adds Rails 6 and 6.1 to Appraisals and fixes Vanity::Rails::Helpers#vanity_js for Rails 6, as in Rails 6+ render file: isn't rendered through ERB anymore. Resolves #390.

Not particularly satisfied with my current solution, but it seemed the simplest one.

[bensheldon]

@ariel-codes thanks for catching that 🎉 Do you feel equipped to tackle those errors, or do you need a hand?

[bensheldon]

Whew! 😓 Took a minute to get to green 🟢

Thank you @ariel-codes for catching the problem and fixing it! 🎉

[bensheldon]

This has been released 🎉 : https://github.com/assaf/vanity/releases/tag/v4.0.3