Update DecimalType.php

The "decimal" mapping type does not map to float/double anymore:
https://github.com/doctrine/dbal/commit/9f51afa210d5c0165dfe6c622e07e42aa54c09fb#lib/Doctrine/DBAL/Types/DecimalType.php

Thank you.
Merge pull request #353 from jbruni/patch-1

Update DecimalType.php