Install missing mod_php under Ubuntu 12.04

Hey,

I've added the package `libapache2-mod-php5` to the `web_apache` recipe in order to make it run unter Ubuntu 12.04.
Without this fix Apache is unable to start and complains about unknown PHP-directives:

> STDERR: Syntax error on line 12 of /etc/apache2/sites-enabled/zabbix.intern.radiologie-lippstadt.de.conf:
> Invalid command 'php_admin_value', perhaps misspelled or defined by a module not included in the server configuration

My patch comes with some specs which can be run with:

```
bundle exec berks install --path vendor/cookbooks && bundle exec rspec
```

Just drop me a line if my PR needs some rework or if something is missing, etc.


Thanks,
Björn

PS: Great cookbook!