Suppress RuboCop offenses

This commit suppresses the following RuboCop offenses:

```console
$ cd path/to/rubocop-ast
$ bundle exec rubocop
(snip)

lib/rubocop/ast/traversal.rb:48:26: C: [Correctable] Performance/StringIdentifierArgument:
Use :"on_#{m}" instead of "on_#{m}".
            alias_method "on_#{m}", "on_#{type}"
                         ^^^^^^^^^
lib/rubocop/ast/traversal.rb:48:37: C: [Correctable] Performance/StringIdentifierArgument:
Use :"on_#{type}" instead of "on_#{type}".
            alias_method "on_#{m}", "on_#{type}"
                                    ^^^^^^^^^^^^

163 files inspected, 2 offenses detected, 2 offenses autocorrectable
```

lib/rubocop/ast/traversal.rb

@@ -45,7 +45,7 @@ def on_#{type}(node)        # def on_send(node)
             end                         # end
           RUBY
           aliases.each do |m|
-            alias_method "on_#{m}", "on_#{type}"
+            alias_method :"on_#{m}", :"on_#{type}"
           end
         end