Fix Meterpreter PHP hop description

If you interpret `File` locations for things in descriptions, you end up with wonky looking paths. For example:

http://www.rapid7.com/db/modules/payload/windows/meterpreter/reverse_hop_http

Incidentally, the canonical way to refer to paths of Metasploit assets are the methods in `Msf::Config`. That's how datastore directories are built, anyway (and still not really appropriate for descriptions).

## Verification Steps

- [x] Start `msfconsole`
- [x] Type `info payload/windows/meterpreter/reverse_hop_http`

## Not addressed

Note that every Meterpreter payload also is including the description from `meterpreter.rb`, for example:

`msf > info payload/windows/meterpreter/reverse_http`

````
Description:
  Tunnel communication over HTTP, Inject the meterpreter server DLL 
  via the Reflective Dll Injection payload (staged)
````

This looks like a bug to me. Will file momentarily.