Compatible with Rails4

Fixed issues with integration in rails4 application.

1. Still use ActiveRecord:: Result instead of Array:

```
categories:
  columns:
  - id
  - category
  - precedence
  - created_at
  - updated_at
  records: 
- !ruby/object:ActiveRecord::Result
  columns:
  - id
  - category
  - precedence
  - created_at
  - updated_at
  rows:
  - - '1'
    - Dentist
    - '1'
```

2. Incorrect type

```
rake aborted!
undefined method `[]=' for #<ActiveRecord::Result:0x007ff50153cc00>
.../yaml_db/lib/serialization_helper.rb:108:in `block in unhash_records'

```