Fix database seeds

Fork no projeto, criei o DB, rodei migrations, e quando rodei `bundle exec rake db:seed`, deu o erro
```
[RailsAdmin] RailsAdmin initialization disabled by default. Pass SKIP_RAILS_ADMIN_INITIALIZER=false if you need it.
rake aborted!
PG::Error: ERROR:  null value in column "created_at" violates not-null constraint
DETAIL:  Failing row contains (1, Arte, null, null, f).
: INSERT INTO categories (name) VALUES ('Arte'),
 ('Artes plásticas'),
 ('Circo'),
 ('Comunidade'),
 ('Feito à mão'),
 ('Humor'),
 ('Quadrinhos'),
 ('Dança'),
 ('Design'),
 ('Eventos'),
 ('Moda'),
 ('Comida'),
 ('Cinema & Vídeo'),
 ('Jogos'),
 ('Jornalismo'),
 ('Música'),
 ('Fotografia'),
 ('Tecnologia'),
 ('Teatro'),
 ('Literatura');

Tasks: TOP => db:seed
(See full trace by running task with --trace)
```