Update katello.conf.erb

Adding support for Scientific Linux
This was weird before, but modern Fedora versions (at least since F19) also ship httpd 2.4 so I don't think this check for Fedora is needed. Even better would be to reuse some logic from the puppetlabs-apache module which has a ```$::apache::apache_version```. They use the following in their templates:
```erb
<%- if scope.function_versioncmp([@apache_version, '2.4']) >= 0 -%>
```

They do bring the variable in scope in the puppet class so you may need to do the same.