Upgrade rspec-rails to 4.0.0.beta2 for Rails 6 support

Author: aldesantis

solidus_auth_devise.gemspec

@@ -42,7 +42,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "ffaker"
   s.add_development_dependency "gem-release", "~> 2.0"
   s.add_development_dependency "github_changelog_generator", "~> 1.14"
-  s.add_development_dependency "rspec-rails", "~> 3.3"
+  s.add_development_dependency "rspec-rails", "~> 4.0.0.beta2"
   s.add_development_dependency "rubocop", "~> 0.71"
   s.add_development_dependency "rubocop-performance", "~> 1.4"
   s.add_development_dependency "rubocop-rails", "~> 2.2"