Tests: Remove Python 2.6 from CI tests

Because:

1. It is old. VERY old.

2. Some useful methods (`assertRaisesRegexp`, `assertIsInstance`) are
   only available from Python 2.7.

.travis.yml

@@ -1,7 +1,6 @@
 ---
 language: python
 python:
-  - 2.6
   - 2.7
   - 3.3
   - 3.4

tests/test_parser.py

@@ -65,8 +65,8 @@ class ParserTestCase(unittest.TestCase):
         e = list(token_generator(''))
         self.assertEqual(len(e), 2)
         self.assertEqual(e[0].prev, None)
-        self.assertTrue(isinstance(e[0].curr, yaml.Token))
-        self.assertTrue(isinstance(e[0].next, yaml.Token))
+        self.assertIsInstance(e[0].curr, yaml.Token)
+        self.assertIsInstance(e[0].next, yaml.Token)
         self.assertEqual(e[1].prev, e[0].curr)
         self.assertEqual(e[1].curr, e[0].next)
         self.assertEqual(e[1].next, None)
@@ -74,20 +74,20 @@ class ParserTestCase(unittest.TestCase):
         e = list(token_generator('---\n'
                                  'k: v\n'))
         self.assertEqual(len(e), 9)
-        self.assertTrue(isinstance(e[3].curr, yaml.KeyToken))
-        self.assertTrue(isinstance(e[5].curr, yaml.ValueToken))
+        self.assertIsInstance(e[3].curr, yaml.KeyToken)
+        self.assertIsInstance(e[5].curr, yaml.ValueToken)
 
     def test_token_or_line_generator(self):
         e = list(token_or_line_generator('---\n'
                                          'k: v\n'))
         self.assertEqual(len(e), 12)
-        self.assertTrue(isinstance(e[0], Token))
-        self.assertTrue(isinstance(e[0].curr, yaml.StreamStartToken))
-        self.assertTrue(isinstance(e[1], Token))
-        self.assertTrue(isinstance(e[1].curr, yaml.DocumentStartToken))
-        self.assertTrue(isinstance(e[2], Line))
-        self.assertTrue(isinstance(e[3].curr, yaml.BlockMappingStartToken))
-        self.assertTrue(isinstance(e[4].curr, yaml.KeyToken))
-        self.assertTrue(isinstance(e[6].curr, yaml.ValueToken))
-        self.assertTrue(isinstance(e[8], Line))
-        self.assertTrue(isinstance(e[11], Line))
+        self.assertIsInstance(e[0], Token)
+        self.assertIsInstance(e[0].curr, yaml.StreamStartToken)
+        self.assertIsInstance(e[1], Token)
+        self.assertIsInstance(e[1].curr, yaml.DocumentStartToken)
+        self.assertIsInstance(e[2], Line)
+        self.assertIsInstance(e[3].curr, yaml.BlockMappingStartToken)
+        self.assertIsInstance(e[4].curr, yaml.KeyToken)
+        self.assertIsInstance(e[6].curr, yaml.ValueToken)
+        self.assertIsInstance(e[8], Line)
+        self.assertIsInstance(e[11], Line)