Rails4 Model.find() fix

This pull request fixes a few deprecation warnings during the tests plus re-enables `Model.find()` behaviour for rails4:

```sql
irb(main):003:0> Job.find 'ad'
  Job Load (0.4ms)  SELECT `jobs`.* FROM `jobs` WHERE `jobs`.`slug` = 'ad' ORDER BY `jobs`.`id` ASC LIMIT 1
# => <Job id: 16 ... >

irb(main):004:0> Job.find 'not-there'
  Job Load (0.4ms)  SELECT `jobs`.* FROM `jobs` WHERE `jobs`.`slug` = 'not-there' ORDER BY `jobs`.`id` ASC LIMIT 1
  Job Load (0.3ms)  SELECT `jobs`.* FROM `jobs` WHERE `jobs`.`id` = 0 LIMIT 1
ActiveRecord::RecordNotFound: Couldn't find Job with id=not-there
```