style(lint): enable default rule and ignore variables with parents

.scss-lint.yml

@@ -17,7 +17,7 @@ linters:
     enabled: false
 
   DefaultRule:
-    enabled: false
+    enabled: true
 
   DuplicateProperty:
     enabled: false

.scss-linters/default_rule.rb

@@ -3,21 +3,15 @@ module SCSSLint
   class Linter::DefaultRule < Linter
     include LinterRegistry
 
-    def visit_each(node)
-      return true
-    end
-
     def visit_function(node)
       return true
     end
 
-    def visit_mixin(node)
-      return true
-    end
-
     def visit_variable(node)
       return if source_from_range(node.source_range).include?('!default')
 
+      return unless node_ancestor(node, 2).nil?
+
       add_lint(node, '!default should be used')
     end
   end