Fix for new ascii reader

Aims at dealing with https://github.com/astropy/astropy/issues/2940 from the astroquery side.
The [error](https://travis-ci.org/astropy/astroquery/jobs/35515577#L1572) is coming from this line, which is strange because it shouldn't be trying to guess.  Perhaps add `guess=False`?  But I suspect a deeper underlying issue is at fault.