Fix `extend Forwardable` for Ruby 2.1.0

```ruby
2.1.0p0 :001 > require 'favicon_maker'
NameError: uninitialized constant FaviconMaker::Generator::Forwardable
	from /Users/leo/.rvm/gems/ruby-2.1.0/gems/favicon_maker-1.1/lib/favicon_maker/generator.rb:12:in `<class:Generator>'
	from /Users/leo/.rvm/gems/ruby-2.1.0/gems/favicon_maker-1.1/lib/favicon_maker/generator.rb:11:in `<module:FaviconMaker>'
	from /Users/leo/.rvm/gems/ruby-2.1.0/gems/favicon_maker-1.1/lib/favicon_maker/generator.rb:3:in `<top (required)>'
	from /Users/leo/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/leo/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/leo/.rvm/gems/ruby-2.1.0/gems/favicon_maker-1.1/lib/favicon_maker.rb:2:in `<top (required)>'
	from /Users/leo/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `require'
	from /Users/leo/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `rescue in require'
	from /Users/leo/.rvm/rubies/ruby-2.1.0/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:144:in `require'
	from (irb):1
	from /Users/leo/.rvm/rubies/ruby-2.1.0/bin/irb:11:in `<main>'

```