gemspec: avoid adding the "lib/" directory to load path

Simply reading the gemspec shouldn't activate the gem by adding its
files to the load path.

guard.gemspec

@@ -1,6 +1,5 @@
 # -*- encoding: utf-8 -*-
-$:.push File.expand_path('../lib', __FILE__)
-require 'guard/version'
+Kernel.load File.expand_path('../lib/guard/version.rb', __FILE__)
 
 Gem::Specification.new do |s|
   s.name        = 'guard'

lib/guard/version.rb

@@ -1,3 +1,3 @@
 module Guard
-  VERSION = "0.4.0"
+  VERSION = "0.4.0" unless defined? VERSION
 end