[Gemspec] Fix for Ruby 2.0.0-p0

See https://bugs.ruby-lang.org/issues/6702

cocoapods.gemspec

 # -*- encoding: utf-8 -*-
 $:.unshift File.expand_path('../lib', __FILE__)
 require 'cocoapods'
+require 'date'
 
 Gem::Specification.new do |s|
   s.name     = "cocoapods"