Changed touch plugin touch_association_value to Time.now instead of CURRENT_TIMESTAMP

hello there

``Sequel::CURRENT_TIMESTAMP`` was wrong because:
- it doesn't match with ``#touch_instance_value`` (one would expect these methods to perform in a similar way)
- it breaks Rails timezone settings, forcing the database setting over Rails
- it breaks ``Timecop`` testing

let me know what you think.

cheers,
Florin.