change service name for compatibility with puppetlabs-apache

Hello,

When writing tests for another module that uses this and puppetlabs-apache I get:

```
     Failure/Error: it { should compile.with_all_deps }
       expected that the catalogue would include Service[apache2] used at /Users/stephenh/Vagrant/vagrant-zabbix/puppet/modules/zabbix/spec/fixtures/modules/php/manifests/apache/config.pp:49 in Php::Config[upload_max_filesize=2M]
```

To fix this I've added a 'httpd' service to php::apache and reference this in php::apache::config.

So far this seems to work fine; the test no longer fails and the same notification takes place if including php::apache when puppetlabs-apache isn't present.