Use RbConfig, move gems from :guard to :development group

Gemfile

@@ -1,18 +1,21 @@
 source :rubygems
 
-# Specify your gem's dependencies in rack-ssl-enforcer.gemspec
 gemspec
 
 gem 'rake'
 
-group :guard do
+require 'rbconfig'
+
+group :development do
   gem 'guard'
   gem 'guard-test'
 
-  if Config::CONFIG['target_os'] =~ /darwin/i
+  if RbConfig::CONFIG['target_os'] =~ /darwin/i
     gem 'rb-fsevent', '~> 0.4'
     gem 'growl'
-  elsif Config::CONFIG['target_os'] =~ /linux/i
+  end
+  
+  if RbConfig::CONFIG['target_os'] =~ /linux/i
     gem 'rb-inotify', '~> 0.5.1'
     gem 'libnotify',  '~> 0.1.3'
   end