Add require 'date', because Date.today requires this

For Ruby2.0.0-rc1
```
$ bundle exec rake -T
There was an error in your Gemfile, and Bundler cannot continue.
$ rake -T
There was an error in your Gemfile, and Bundler cannot continue.
```

carrierwave.gemspec

@@ -3,6 +3,7 @@ lib = File.expand_path('../lib/', __FILE__)
 $:.unshift lib unless $:.include?(lib)
 
 require 'carrierwave/version'
+require 'date'
 
 Gem::Specification.new do |s|
   s.name = "carrierwave"