Fix bug when exporting to file in current folder
Fix bug when exporting to file in current folder

In that case, `os.path.dirname('foo.json')` returns an empty string,
then `os.path.exists('')` returns True, and it tries to call
`os.makedirs('')`, which raises an IOError.
okfn/datapackage-sql-py,okfn/datapackage-sql-py