Increase default pool_size to 4

Solves #1038.  pool_size was added as an optional setting
a while ago and people have been using it successfully
in the wild.  This commit increases the default to 4
which seems like a generally useful improvement.

lib/r10k/puppetfile.rb

@@ -10,7 +10,7 @@ class Puppetfile
 
   include R10K::Settings::Mixin
 
-  def_setting_attr :pool_size, 1
+  def_setting_attr :pool_size, 4
 
   include R10K::Logging
 

lib/r10k/settings.rb

@@ -159,8 +159,8 @@ def self.global_settings
         }),
 
         Definition.new(:pool_size, {
-          :desc => "The amount of threads used to concurrently install modules. The default value is 1: install one module at a time.",
-          :default => 1,
+          :desc => "The amount of threads used to concurrently install modules. The default value is 4: install four modules at a time.",
+          :default => 4,
           :validate => lambda do |value|
             if !value.is_a?(Integer)
               raise ArgumentError, "The pool_size setting should be an integer, not a #{value.class}"