Fix internal validations for consistency and completeness

The validations for consistency and completeness in BagIt::Bag were swapped:

```ruby
validates_true_for :consistency, :logic => Proc.new { complete? }
validates_true_for :completeness, :logic => Proc.new { consistent? }
```

The RSpec examples did not catch this because they ran all validations before checking for errors.  Using validate_only before checking for errors fixes this.

```ruby
@bag.validate_only('true_for/completeness')
@bag.errors.on(:completeness).should_not be_empty
@bag.should_not be_valid
```