Fix uninitialized constant Parser::AST::Processor::Mixin

parser 3.3.1.0 introduced
whitequark/parser#1000, which causes this
failure:

```
% bundle exec ruby -Itest test/unit/profiles/profile_test.rb
inspec/lib/inspec/utils/profile_ast_helpers.rb:7:in `<class:CollectorBase>': uninitialized constant Parser::AST::Processor::Mixin (NameError)

        include Parser::AST::Processor::Mixin
                                      ^^^^^^^
```

Fix this by inherting from `Parser::AST::Processor` and
requiring the right version of the `parser` gem.

Closes inspec#7029

inspec.gemspec

@@ -54,5 +54,5 @@ Source code obtained from the Chef GitHub repository is made available under Apa
   spec.add_dependency "train-kubernetes", "~> 0.1"
 
   spec.add_dependency "mongo", "= 2.13.2" # 2.14 introduces a broken symlink in mongo-2.14.0/spec/support/ocsp
-
+  spec.add_dependency "parser", ">= 3.3.1.0" # 3.3.1.0 removed a mix-in
 end

lib/inspec/utils/profile_ast_helpers.rb

@@ -3,8 +3,7 @@
 module Inspec
   class Profile
     class AstHelper
-      class CollectorBase
-        include Parser::AST::Processor::Mixin
+      class CollectorBase < Parser::AST::Processor
         include RuboCop::AST::Traversal
 
         attr_reader :memo