[1.7.x] Skipped JavascriptI18nTests if Firefox isn't properly configured.

Code borrowed from django.contrib.admin.tests. Without this, the class can
throw an exception with something like "The browser appears to have exited
before we could connect. The output was: Error: no display specified"

Backport of e2ac0203 from master

tests/view_tests/tests/test_i18n.py

@@ -11,13 +11,9 @@ from django.test import (
     LiveServerTestCase, TestCase, modify_settings, override_settings)
 from django.utils import six
 from django.utils._os import upath
+from django.utils.module_loading import import_string
 from django.utils.translation import override, LANGUAGE_SESSION_KEY
 
-try:
-    from selenium.webdriver.firefox import webdriver as firefox
-except ImportError:
-    firefox = None
-
 from ..urls import locale_dir
 
 
@@ -211,16 +207,20 @@ skip_selenium = not os.environ.get('DJANGO_SELENIUM_TESTS', False)
 
 
 @unittest.skipIf(skip_selenium, 'Selenium tests not requested')
-@unittest.skipUnless(firefox, 'Selenium not installed')
 class JavascriptI18nTests(LiveServerTestCase):
 
     # The test cases use translations from these apps.
     available_apps = ['django.contrib.admin', 'view_tests']
     urls = 'view_tests.urls'
+    webdriver_class = 'selenium.webdriver.firefox.webdriver.WebDriver'
 
     @classmethod
     def setUpClass(cls):
-        cls.selenium = firefox.WebDriver()
+        try:
+            cls.selenium = import_string(cls.webdriver_class)()
+        except Exception as e:
+            raise unittest.SkipTest('Selenium webdriver "%s" not installed or '
+                                    'not operational: %s' % (cls.webdriver_class, str(e)))
         super(JavascriptI18nTests, cls).setUpClass()
 
     @classmethod