Delete CartoDB::Importer2::Downloader temp files

During the import process, the Downloader class can call
Unp.new.generate_temporary_directory.temporary_directory to create a
temporary directory to store downloaded files. However, this temporary
folder doesn't get deleted after the import process completes, which can
cause the machine running resque tasks to run out of disk space
eventually.

This adds a clean_up method to CartoDB::Importer2::Downloader which is
called after imports complete to remove the temporary folder created by
the downloader.