Require StateFile if needed.

lib/puma.rb

@@ -1,7 +1,6 @@
 # Standard libraries
 require 'socket'
 require 'tempfile'
-require 'yaml'
 require 'time'
 require 'etc'
 require 'uri'

lib/puma/launcher.rb

@@ -7,7 +7,6 @@
 require 'puma/const'
 
 require 'puma/binder'
-require 'puma/state_file'
 
 module Puma
   # Puma::Launcher is the single entry point for starting a Puma server based on user
@@ -105,6 +104,8 @@ def write_state
       path = @options[:state]
       return unless path
 
+      require 'puma/state_file'
+
       sf = StateFile.new
       sf.pid = Process.pid
       sf.control_url = @options[:control_url]

test/test_cli.rb

@@ -2,6 +2,7 @@
 require 'test/unit'
 require 'puma/cli'
 require 'tempfile'
+require 'yaml'
 
 class TestCLI < Test::Unit::TestCase
   def setup