Prevent an attempt of adding should for RSpec 4

Author: pirj

lib/rspec/rails/extensions/active_record/proxy.rb

@@ -1,7 +1,10 @@
 RSpec.configure do |rspec|
   # Delay this in order to give users a chance to configure `expect_with`...
   rspec.before(:suite) do
-    if defined?(RSpec::Matchers) && RSpec::Matchers.configuration.syntax.include?(:should) && defined?(ActiveRecord::Associations)
+    if defined?(RSpec::Matchers) &&
+        RSpec::Matchers.configuration.respond_to?(:syntax) && # RSpec 4 dropped support for monkey-patching `should` syntax
+        RSpec::Matchers.configuration.syntax.include?(:should) &&
+        defined?(ActiveRecord::Associations)
       RSpec::Matchers.configuration.add_should_and_should_not_to ActiveRecord::Associations::CollectionProxy
     end
   end