Skip directories when collecting artifacts

We're frequently getting messages like:

```
2014-06-23 20:20:38 [ERROR] Error uploading artifact /opt/buildbox/builds/envato-marketplaces/marketplace/results/features (read /opt/buildbox/builds/envato-marketplaces/marketplace/results/features: is a directory)
```

And then the directories still appear in the artifacts list and can't be viewed (because they haven't been uploaded).

This skips directories during the collection phase.