Make tox pass selenium environment variables

When running tox, pass through the user's DISPLAY and
DJANGO_SELENIUM_TESTS environment variables, so that

    DJANGO_SELENIUM_TESTS=true tox

will actually run the Selenuim integration tests.  Without this change,
the test suite never sees the DJANGO_SELENIUM_TESTS variable and thus
skips the integration tests.  Without DISPLAY, the integration tests
will error out (unless CI is present in the environment to instruct the
test suite to run the Selenium webdriver in headless mode).

tox.ini

@@ -30,6 +30,8 @@ passenv=
     DB_PASSWORD
     DB_HOST
     DB_PORT
+    DISPLAY
+    DJANGO_SELENIUM_TESTS
     GITHUB_*
 setenv =
     PYTHONPATH = {toxinidir}