Bug 1348585 - Add six to the mach virtualenv path. r=chmanchester

Ensure that six is available when running python-test tests, which
have the virtualenv python environment not that of the parent mach
process.

MozReview-Commit-ID: FrwzPZvMCSI

--HG--
extra : rebase_source : 3ce5f9d85baf8064fc547f4cd5fb94f843f7186d

build/virtualenv_packages.txt

@@ -2,6 +2,7 @@ altgraph.pth:python/altgraph
 marionette_driver.pth:testing/marionette/client
 marionette_harness.pth:testing/marionette/harness
 browsermobproxy.pth:testing/marionette/harness/marionette_harness/runner/mixins/browsermob-proxy-py
+six.pth:testing/web-platform/tests/tools/six
 wptserve.pth:testing/web-platform/tests/tools/wptserve
 blessings.pth:python/blessings
 configobj.pth:python/configobj