Update montage.py

Using Python3 and the dev version of MNE, I get an error trying to read a montage (.sfp), in line 147/148 of mne/montages/montage.py:
```ValueError: need more than 0 values to unpack```

This is possibly because ch_names_ is a list of items of type S4 and ch_names is a list of strings (see line 100-103).
I get no errors if I add a conversion of ch_names_ to np.string (see line 103).

Don't know if this breaks Python2 or anything further down in the processing pipeline though.