Use parser 2.4.0.0

This matches the version that supports Ruby 2.3.3 syntax (which is the
version we use).

Library/Homebrew/cmd/style.rb

@@ -70,6 +70,7 @@ module Homebrew
   def check_style_impl(files, output_type, options = {})
     fix = options[:fix]
 
+    Homebrew.install_gem_setup_path! "parser", HOMEBREW_RUBOCOP_PARSER_VERSION, "ruby-parse"
     Homebrew.install_gem_setup_path! "rubocop", HOMEBREW_RUBOCOP_VERSION
     require "rubocop"
     require_relative "../rubocops"

Library/Homebrew/constants.rb

@@ -2,4 +2,5 @@
 
 # RuboCop version used for `brew style` and `brew cask style`
 HOMEBREW_RUBOCOP_VERSION = "0.51.0"
+HOMEBREW_RUBOCOP_PARSER_VERSION = "2.4.0.0" # for Ruby 2.3.3
 HOMEBREW_RUBOCOP_CASK_VERSION = "~> 0.15.1" # has to be updated when RuboCop version changes

Library/Homebrew/test/Gemfile

@@ -3,6 +3,7 @@ source "https://rubygems.org"
 require_relative "../constants"
 
 gem "parallel_tests"
+gem "parser", HOMEBREW_RUBOCOP_PARSER_VERSION
 gem "rspec"
 gem "rspec-its", require: false
 gem "rspec-wait", require: false

Library/Homebrew/test/Gemfile.lock

@@ -58,6 +58,7 @@ PLATFORMS
 DEPENDENCIES
   codecov
   parallel_tests
+  parser (= 2.4.0.0)
   rspec
   rspec-its
   rspec-wait
@@ -65,4 +66,4 @@ DEPENDENCIES
   simplecov
 
 BUNDLED WITH
-   1.15.4
+   1.16.0