Change {image}_updated_at to Time (was DateTime)

[johnnyshields]

For {image}_updated_at field, type Time is better to use than DateTime.

• It has better performance
• It's what Mongoid uses for created_at / updated_at timestamps.
• This change can be done without needing to migrate the data in the database.
• For the vast majority of apps this will not have an real impact, since ActiveSupport standardizes the main methods of Time/DateTime.

[dshoemaker]

@mrrooijen any chance this change could get an approval? We're leveraging this gem and attempting to follow Ruby's soft deprecation of DateTime in favor of Time.