reworked fix for #669 (the infamous test-unit bundle conflict)

In case if we need test-unit-1.2.3 for anything else than Rails2/CP.
(if not, just change Gemfile condition to "and chiliproject")

This reverts commit 0cd902610fb6a8d7fdc6c11c783d085cfb9adb32, and adds alternative solution.

Also see discussion: http://forum.redminebacklogs.net/can-t-cleanly-install-RBL-quot-same-gem-twice-with-different-version-requirements-quot-tc4025469.html