Bug from 8f62f16: Unix domain socket; ERR invalid DB index

````
var kue = require('kue');

kue.createQueue({
redis: {
socket: '/path/to/redis_sock',
},
});
````

throws an unhandled exception: "ReplyError: ERR invalid DB index"

This is due to changes in lib/redis.js introduced in 8f62f16:

Line 114: `` var db = !socket ? (options.redis.db || 0) : null;``

Line 119-121: `` if( db >= 0 ){ client.select(db); }``

Since ``(null >= 0) === true`` , ``client.select(null)`` will be run when initiating the redis connection over a socket path, resulting in the mentioned error. I made the pull request #999 a while back to fix this. A similar solution would be to change line 114 to `` var db = !socket ? (options.redis.db || 0) : undefined;`` but I was not sure if setting to undefined would be accepted as it seems to go against the programming style. If you prefer ``db = undefined``, let me know and I'll amend my PR.

When I submitted this pull request, I was mistakenly thinking, that #396 was referencing the same issue, now I realize that the bug was not introduced until October 2015. Please merge this request, as unix domain sockets are quite useful on shared hosting environments.

Start in lib/redis.js at lines 114 and 119-121, then reproduce the queue setup using a Redis Unix socket. Verify that connecting through the socket no longer calls select with an invalid database index and no unhandled ERR invalid DB index exception occurs.

Written by the indexing model from the issue text.