Added a test to complete utils coverage.

tests/test_rules_utils.py

@@ -88,6 +88,7 @@ class TestRulesUtils(RulesTestCase):
         self.assertTrue(is_prime(2))
         self.assertTrue(is_prime(3))
         self.assertFalse(is_prime(6))
+        self.assertFalse(is_prime(9))
         self.assertTrue(is_prime(19))
         self.assertTrue(is_prime(43))