Fix path.resolve error in newer node versions

Running doks-server on newer versions of node throws a TypeError

```
TypeError: Arguments to path.resolve must be strings
    at exports.resolve (path.js:313:15)
    at /home/docs/doks-server/node_modules/underscore/underscore.js:593:19
    at Array.map (native)
    at Function._.map._.collect (/home/docs/doks-server/node_modules/underscore/underscore.js:97:56)
    at DocSetGroup.docsetPaths (/home/docs/doks-server/lib/docsetgroup.js:38:14)
    at DocSetGroup.open (/home/docs/doks-server/lib/docsetgroup.js:51:18)
    at Server.open (/home/docs/doks-server/lib/server.js:48:26)
    at main (/home/docs/doks-server/bin/doks-server:26:12)
    at Object.<anonymous> (/home/docs/doks-server/bin/doks-server:33:1)
    at Module._compile (module.js:456:26)
```
This is caused by `map` passing the index and list to resolver. Old node versions just ignored non-string parameters, so this was ok. But newer node versions force all arguments to be strings.