Change REGEX to REGEXP

MySQL does not have expression called `REGEX`. Correct name is [REGEXP](http://dev.mysql.com/doc/refman/4.1/en/regexp.html). Before PR the following code dies with error:

```php
$mapper->where(["title :regex" => "_foo$"])
```
```
PDOException: SQLSTATE[42000]: Syntax error or access violation: 1064
```