travis: Ensure we use pytest 2.8.7

The latest version of pytest (4.0.0) makes some of the code we have in
test/py/conftest.py a fatal error that needs to be migrated.
Unfortunately this in turn requires changes that don't exist in older
versions of pytest such as 2.8.7 that ships with Ubuntu 16.04.  Force
travis to use this older version of pytest.

Signed-off-by: Tom Rini <trini@konsulko.com>