Change default to_time of distance_of_time_in_words to Time.now

Note : If someone from the core team is okay with this, I'll then make a Changelog entry and tests.

Most of the time you use ``distance_of_time_in_words``, you deal with ``Time.now``

Naively I tried to do :

````ruby
distance_of_time_in_words(2.hours.from_now)
=> "More than 44 years"
````

Not very intuitive right ?