Use the 6-0-stable version of Rails to fix issue with JSON responses

The test suite was failing on Rails 6.0 + Ruby 3 with errors like:

    Expected "{\"errors\":\"#<ActiveModel::Errors:0x000055f2e6cb8188>\"}"
    to include "{\"errors\":{".

The ActiveModel::Errors object wasn't being serialized to JSON as
expected, and this only happened with that combination of Ruby/Rails.

Upon further investigation, this was caused by a change in Ruby and
fixed in Rails in this PR: rails/rails#39697
(which describes in more details the exact same problem and links to the
Ruby bug tracker with more information).

That fix was backported to 6-0-stable in June 2020, but hasn't been
officially released in a stable version yet: (there have been only
security fixes since then for 6.0)
rails/rails@75f6539

Since the branch contains the fix, I'm pointing directly to it to get
the tests passing. We can't tell if there'll be a new stable 6.0 release
at this point, but hopefully yes, in which case we can go back at
pointing to it.

gemfiles/Gemfile-rails-6-0

@@ -2,7 +2,7 @@ source "https://rubygems.org"
 
 gemspec path: ".."
 
-gem "rails", '~> 6.0.0'
+gem "rails", '~> 6.0.0', github: 'rails/rails', branch: '6-0-stable'
 gem "omniauth"
 gem "omniauth-oauth2"
 gem "rdoc"