Remove call to `#to_h` as it produces an error 1.9.3p429

I am running Sequel::Fixture with Ruby 1.9.3p429. When the call to `#table_data.rows` gets executed, it returns a simple list of hashes. In 1.9.3 instances of Hash do not respond to `#to_h` (see [Ruby doc](http://www.ruby-doc.org/core-1.9.3/Hash.html)). Is there any specific reason for that method call? If there is, perhaps I can amend the PR so that it will fallback to `#to_hash` where `#t_h` is not available.