stbt.ocr: hide Tesseract use message

With every call, Tesseract prints something like
"""
Tesseract Open Source OCR Engine v3.02.02 with Leptonica
"""
to stderr. This is really annoying when running a test as `stbt run`
(i.e. no -v flag) because the Tesseract messages come through and messes
up my regular debug (e.g. printing what Tesseract has read).

This patch does have the downside of sending *all* stderr from Tesseract
to /dev/null, however the extra work necessary to create piped processes
in Python makes this this method more desirable.

From a user's perspective, if there is a problem with the Tesseract call,
they get an exception because (we are using `check_call`) that makes it
very clear the problem was with the Tesseract call. Moreover, the only
kind of errors they are likely to get are that there is a problem with
the installation - the whole call to Tesseract is tightly controlled by
stbt and there aren't many ways for it to go wrong.