Require date when time is required

In MRI 1.9 and 2.0 there is ```require 'date'``` inside time.rb. 
https://github.com/ruby/ruby/blob/ruby_1_9_3/lib/time.rb#L43
https://github.com/ruby/ruby/blob/trunk/lib/time.rb#L1

So following works:

```ruby
  require 'time'
  Date.today
```