Fix JS regression in `app.import`.

We are simply using `pickFiles` here to ensure that the specific
directory is watched, we do not actually care about the destination
files.

Without this change specifying/modifying `vendor/` and passing it into
`EmberApp` would throw an error if the actual file did not exist on disk
Merge pull request #705 from rjackson/ensure-watched-directories-dont-blow

Fix JS regression in `app.import`.