update rubocop configuration for CI

Author: rei-moo

.rubocop.yml

@@ -2,3 +2,19 @@ inherit_gem:
   rubocop-github:
     - config/default.yml
 require: rubocop-performance
+
+AllCops:
+  TargetRubyVersion: 2.6
+
+# Disable cops that are not consistently available across all Ruby versions
+Style/ClassMethodsDefinitions:
+  Enabled: false
+
+Style/OrAssignment:
+  Enabled: false
+
+Layout/SpaceInsideHashLiteralBraces:
+  Enabled: false
+
+Lint/ParenthesesAsGroupedExpression:
+  Enabled: false

spec/lib/secure_headers/configuration_spec.rb

@@ -18,11 +18,13 @@ module SecureHeaders
 
     it "dup results in a copy of the default config" do
       Configuration.default
-      original_configuration = Configuration.send(:default_config)
+      original_configuration = Configuration.public_send(:default_config)
       configuration = Configuration.dup
       expect(original_configuration).not_to be(configuration)
       Configuration::CONFIG_ATTRIBUTES.each do |attr|
-        expect(original_configuration.send(attr)).to eq(configuration.send(attr))
+        # rubocop:disable GitHub/AvoidObjectSendWithDynamicMethod
+        expect(original_configuration.public_send(attr)).to eq(configuration.public_send(attr))
+        # rubocop:enable GitHub/AvoidObjectSendWithDynamicMethod
       end
     end