Mordernize code base with __dir__ and require_relative

lib/liquid.rb

@@ -73,4 +73,4 @@ require 'liquid/token'
 
 # Load all the tags of the standard library
 #
-Dir[File.dirname(__FILE__) + '/liquid/tags/*.rb'].each { |f| require f }
+Dir["#{__dir__}/liquid/tags/*.rb"].each { |f| require f }

lib/liquid/i18n.rb

@@ -2,7 +2,7 @@ require 'yaml'
 
 module Liquid
   class I18n
-    DEFAULT_LOCALE = File.join(File.expand_path(File.dirname(__FILE__)), "locales", "en.yml")
+    DEFAULT_LOCALE = File.join(File.expand_path(__dir__), "locales", "en.yml")
 
     TranslationError = Class.new(StandardError)
 

lib/liquid/tags/unless.rb

@@ -1,4 +1,4 @@
-require File.dirname(__FILE__) + '/if'
+require_relative 'if'
 
 module Liquid
   # Unless is a conditional just like 'if' but works on the inverse logic.