remove schema timestamps from Groups and GroupPermissions

The timestamps causes inserts of Groups and GroupPermissions to fail in strict mode on MySQL and on SQLite

```
SQLSTATE[23000]: Integrity constraint violation: 19 groups.created_at may not be NULL (SQL: insert into "groups" ("id", "title") values (?, ?)) (Bindings: array ( 0 => 1, 1 => 'seed_data.administrators', )) 
```

Either the timestamp fields need to be removed (they aren't being used by the models) or the models should have them added like so:
```
public $timestamps = array('created_at', 'updated_at');
```