Don't pass exec's stderr to the callback.

This fixes an error during rollback where the result of several exec's is expected to be [ 'path_to_file\n' ]
but instead is returned as [ [ 'path_to_file\n', '' ] ].
Add test for handling childProcess.exec callback
Merge pull request #7 from hellosimon/master

Don't pass exec's stderr to the callback.