[#106] Fix wrong inifile setting for xcache extension.

```bash
==> default: Error: /Stage[main]/[...]: Could not evaluate: Save failed with return code false, see debug
```

The error occurs if you try to set settings for xcache extension.

Actual: ``$inifile = '/etc/php5/conf.d/20-xcache.ini'``
Should be: ``$inifile = "${php::params::config_root_ini}/xcache.ini"``

This PR fixes this issue (#106).