Basic fix for issue #1413

The 'dir' variable is not handed to the callback as a string, in every
Windows installation of 1.3.8 it is being returned as an array because
of the tmp dependency being updated. The details of this failure are
highlighted in issue #1413 and is a major blocker in using Bower
successfully on Windows.

Pull request #1407 can be ignored if this code fix is in place instead
which supports both return variations of tmp.dir.