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.
```

```Data.today``` in carrierwave.gemspec brings this.
Simular to this SO:
http://stackoverflow.com/questions/9704969/why-is-rubys-date-class-automatically-loaded-but-datetime-is-not