Add exclude credential master key of Rails 5.2

Rails.gitignore

@@ -14,6 +14,7 @@ pickle-email-*.html
 
 # TODO Comment out this rule if you are OK with secrets being uploaded to the repo
 config/initializers/secret_token.rb
+config/master.key
 
 # Only include if you have production secrets in this file, which is no longer a Rails default
 # config/secrets.yml