rubocop: move requirement to configuration file

If you are using en external rubocop binary you will encounter the
following error.

.rubocop_todo.yml: RSpec/FilePath has the wrong namespace - should be Rails

By moving the option to the config file an external rubocop would be
treated equally to  brew style  without having to supply additional
command line options.

Library/.rubocop.yml

@@ -5,7 +5,9 @@ AllCops:
     - '**/vendor/**/*'
   DisplayCopNames: false
 
-require: ./Homebrew/rubocops.rb
+require: 
+  - rubocop-rspec
+  - ./Homebrew/rubocops.rb
 
 # enable all formulae audits
 FormulaAudit:

Library/Homebrew/style.rb

@@ -23,7 +23,6 @@ module Homebrew
       require "rubocops"
 
       args = %w[
-        --require rubocop-rspec
         --force-exclusion
       ]
       if fix