Relax rspec-rails version requirement

Rails 7.2 removed the `fixture_path=` method which older rspec-rails
versions rely on. Allowing rspec-rails >= 3.6 lets Bundler pick a
compatible version for each Rails version in the test matrix.

Author: tf

activeadmin-searchable_select.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler', '>= 1.5'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'appraisal', '~> 2.2'
-  spec.add_development_dependency 'rspec-rails', '~> 3.6'
+  spec.add_development_dependency 'rspec-rails', '>= 3.6'
   spec.add_development_dependency 'combustion', '~> 1.0'
   spec.add_development_dependency 'database_cleaner-active_record', '~> 2.1'
   spec.add_development_dependency 'sqlite3', '~> 1.3'