add filter so make_dataset.py ignores hidden files in input_dir

`python src/data/make_dataset.py raw_output_dir processed test_output -doo -dlo`

spotted this
```2018-11-01 11:56:54,162 - root - INFO - read_file  Reading: /Users/matthewgregory/Documents/gds/govuk-network-data/data/raw_output_dir/.DS_Store```
should hidden files like this be being read?

code fails
OSError: Not a gzipped file (b’\x00\x00')