Rails 6.1 compatibility

[Capncavedan]

Address deprecation warning about single-arity template handlers. This is the same as how the jbuilder project handled it.

DEPRECATION WARNING: Single arity template handlers are deprecated. Template handlers must
now accept two parameters, the view object and the source for the view object.
Change:
  >> #<ActionView::Template::Handlers::RJS:0x00007ff7621df338>.call(template)
To:
  >> #<ActionView::Template::Handlers::RJS:0x00007ff7621df338>.call(template, source)
 (called from <top (required)> at /Users/danb/.rvm/gems/ruby-2.5.7@exceedlms/bundler/gems/prototype-rails-e8dd3cd4e722/lib/prototype-rails/on_load_action_view.rb:24)

[Capncavedan]

I had to run bundle update mimemagic as several older versions have been yanked

[Capncavedan]

@aud-tree you've worked in this forked gem a bit - mind just giving this a quick look?