.decrement(...) returns an empty array rather than # of rows affected

As we talked about in IRC, decrement is returning an empty array rather than a count of rows affected.

```
db('a_table')                                         
  .decrement('credits', 1)
  .where('credits', '>=', 1)
  .andWhere('id', 'in', [1,2,3])
  .exec(function(err, result) {
    // result is an empty array rather than a number
  });      
```
