travis: Fix allowing more CPU usage

The original commit[1] was not enough because we run tests using tox and
tox by default doesn't pass all environment variables to the
environments in which tests run - this patch whitelists subset of
environment variables set by Travis, including one that we read and use.

[1] 74f418bcf3

tox.ini

@@ -28,6 +28,7 @@ commands =
     pep8 benchmarks/ eventlet/ tests/
 
 [testenv]
+passenv = TRAVIS*
 setenv =
     PYTHONDONTWRITEBYTECODE = 1
     selects: EVENTLET_HUB = selects