Merge pull request #633 from presidentbeef/fix_compare_and_external_checks

Fix using --compare and --add-checks-path together

lib/brakeman.rb

@@ -399,8 +399,6 @@ module Brakeman
     require 'brakeman/differ'
     raise ArgumentError.new("Comparison file doesn't exist") unless File.exists? options[:previous_results_json]
 
-    add_external_checks options
-
     begin
       previous_results = MultiJson.load(File.read(options[:previous_results_json]), :symbolize_keys => true)[:warnings]
     rescue MultiJson::DecodeError