Drop using microseconds for migration versions

Consider a case where you specify a migration that's was created later as a target (`rake db:migrate VERSION=xxxx`), for AR to decide whether to migrate up or down, it would get the maximum value found in the schema_migrations table, which would one of the gem's three migration files due to the presence of the extra 6 digits, AR would consider it a **down** migration, reverting (or dropping!) translation_center tables.

Inhering from `ActiveRecord::Generators::Base` is preferable because it handles versions conflicts correctly. This leaves us with acts_as_votable migration version conflicts with ours, so we use a little meta-programming (read, hack  :smile: ). I removed the `sleep(1)` call because it's not needed anymore (audited plays nice with the rest), besides, time is of the essence.