Fix tox.ini for both tox 1.6 and 1.7

The current way fails to work with tox 1.6 when building the translated
manuals.
Fix so that it now really works with both tox 1.6 and 1.7.

Note that this is a workaround for a bug in tox 1.7.

Change-Id: Ib1cf8ccfc162fe313b9f2be35b78f5004634f97b