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.
```
Merge pull request #953 from sanemat/fix/ruby2-0-0-gemspec

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