Fix issue where plugin fails if directory has a `.`

Looks like the method for determining whether or not something is a file is if it matches the pattern `'**/*.*'`.  Unfortunately, directories that have a `.` in them also match this pattern, as I noticed on a project I was working on. The specific example I saw where this was failing was on:

```
bower_components/accounting.js/accounting.js
```

I believe the included method will be more robust. the `walk-sync` library guarantees that all directories will end in a `/`. This is the method [used in broccoli-filter](https://github.com/broccolijs/broccoli-filter/blob/master/index.js#L36) for instance.