Fix for pytest 3.3.0
Fix for pytest 3.3.0

Fixes https://github.com/abusalimov/pytest-travis-fold/issues/4.

The `writer` property was removed again in pytest 3.3.0
(https://github.com/pytest-dev/pytest/commit/05cfdcc8).

This reverts "Replace usage of 'terminalreporter._tw' -> '.writer'"
(commit 25bf3cc1fa2e5da0d87985b1975b21ceb449d19d).

abusalimov/pytest-travis-fold