Fixed issue loading DBMySQLi.php in propel in sf1.3 and sf1.4

Somewhere after Symfony1.1, they removed DBMySQLi.php from the propel adapters. To fix the class/file not found issues, I have reverted it back to mysql as driver. A proper fix might be required. 

```
FatalErrorException: Compile Error: require(): Failed opening required 'propel/adapter/DBMySQLi.php' 
```