Use require_relative where possible

Also solves #1412

mail.gemspec

@@ -1,4 +1,4 @@
-require './lib/mail/version'
+require_relative 'lib/mail/version'
 
 Gem::Specification.new do |s|
   s.name        = "mail"

rakelib/corpus.rake

@@ -3,7 +3,7 @@ require 'benchmark'
 namespace :corpus do
 
   task :load_mail do
-    require File.expand_path('../../spec/environment', __FILE__)
+    require_relative '../spec/environment'
     require 'mail'
   end
 

rakelib/generate_tables

@@ -4,7 +4,7 @@
 # Usage:
 #   ruby tasks/generate_tables
 # The Unicode version downloaded is determined by the UNICODE_VERSION lib/mail/multibye/unicode.rb
-require './lib/mail/multibyte/unicode'
+require_relative '../lib/mail/multibyte/unicode'
 
 require 'open-uri'
 require 'tmpdir'

spec/spec_helper.rb

@@ -1,6 +1,7 @@
 # encoding: utf-8
 # frozen_string_literal: true
-require File.expand_path('../environment', __FILE__)
+
+require_relative 'environment'
 
 unless defined?(MAIL_ROOT)
   $stderr.puts("Running Specs under Ruby Version #{RUBY_VERSION}")
@@ -17,7 +18,7 @@
 end
 
 require 'rspec'
-require File.join(File.dirname(__FILE__), 'matchers', 'break_down_to')
+require_relative 'matchers/break_down_to'
 
 require 'mail'