fix error where grunt recurse tries to use path.join with undefined variables

Hi,
When trying to run grunt (or indeed just installing this from npm) I found that inside grunt.file.recurse a call was being made to path.join with an undefined value. Setting the subdir parameter prevents this from occurring.

I did look at upgrading this to a newer version of grunt but it seems to be non trivial compared to this solution.

Cheers,
Derek