Exclude ast.rb from Rubocop

Because ast.rb automatically generated from
puppet/tasks/generate_ast_model.rake, this commit excludes it from all
Rubocop checks.

Author: mhashizume

.rubocop.yml

@@ -15,6 +15,7 @@ AllCops:
     - 'tasks/**/*'
     - 'ext/suse/puppet.spec'
     - 'lib/puppet/vendor/**/*'
+    - 'lib/puppet/pops/model/ast.rb'
     - 'lib/puppet/pops/parser/eparser.rb'
 
 # The formatting of defaults is unusual, so let's skip layout cops.