[performance] Disable advanced layout analysis

[jstockwin]

I noticed that by setting boxes_flow outside the documented range, you can actually disable PDFMiner's advanced layout analysis.

We don't need the advanced analysis since we have no hierarchy of text boxes and we order them ourselves, and it's quite a performance gain to leave these out.

I've filed an issue (and fix) to update the documentation and also allow boxes_flow to be passed as None to explicitly disable this: pdfminer/pdfminer.six#395

Once that's merged, we should either default or hard-code our boxes_flow la param to None. It feels like we should allow it to be overridden, but equally since we ignore the resulting analysis perhaps there's no point and we should hard-code it to None.

[jstockwin]

There's a new pdfminer release (20200517) which has a fix for the blocking pdfminer/pdfminer.six#411 bug.

We should bump the pdfminer version and then go ahead and implement the above.