FIX _write_raw():  don’t set meas_id to invalid value

I've encountered a bug with kit2fiff, but I think it's a bug in the general write raw code:

 - if `raw.info['meas_id']` is `None` (as it was for my kit-raw file), `_write_raw()` sets `meas_id = 0` before calling `write_id(fid, FIFF.FIFF_REF_FILE_ID, meas_id)` (https://github.com/mne-tools/mne-python/blob/master/mne/io/base.py#L1449)
 - `write_id()` however expects `meas_id` to be a dictionary (https://github.com/mne-tools/mne-python/blob/master/mne/io/write.py#L181)

Since `write_id()` seems to be able to handle `meas_id == None` I think this PR should solve it, currently running the tests...