Lock version of concurrent-ruby to avoid issue with rails 7.0 (joshwlewis#60)

Author: codez

spec/ci/rails70.gemfile

@@ -4,6 +4,8 @@ gemspec path: '../..'
 
 gem 'sqlite3', '< 2.0'
 
+gem 'concurrent-ruby', '1.3.4' # required because of https://github.com/rails/rails/issues/54260
+
 gem 'simplecov', require: false
 gem 'simplecov-lcov', require: false