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: https://github.com/rails/rails/pull/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)
75f6539d0e

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"