Fix issue with PHP_VERSION on Ubuntu servers

Hi,

I run Magento on Ubuntu server, I installed the php provided in the distribution packages. But this php package add a suffix to the PHP_VERSION constant

```bash
$ php -r 'echo PHP_VERSION, "\n";'
5.3.10-1ubuntu3.4
```

This suffix make the version_compare test performed during the installation of a Magento module faill. 

```bash
$ php -r 'var_dump(version_compare(PHP_VERSION, "5.3.10", "<="));'
bool(false) // but it should be true
```

This quick patch extract the real version of the PHP_VERSION string

```bash
$ php -r 'echo substr(PHP_VERSION, 0, strpos(PHP_VERSION, "-")), "\n";'
5.3.10

$ php -r 'var_dump(version_compare(substr(PHP_VERSION, 0, strpos(PHP_VERSION, "-")), "5.3.10", "<="));'
bool(true)
```

and allow me to install the module I need.