Fix broken test for the invalid notebook

IPython/nbformat/tests/invalid.ipynb

@@ -121,10 +121,7 b''
     {
      "cell_type": "heading",
      "level": 3,
-     "metadata": {},
-     "source": [
-      "Image"
-     ]
+     "metadata": {}
     },
     {
      "cell_type": "code",

IPython/nbformat/tests/test_validator.py

@@ -48,7 +48,7 b' class TestValidator(TestsBase):'
         """Test than an invalid notebook does not pass validation"""
         # this notebook has a few different errors:
         # - the name is an integer, rather than a string
-        # - one cell is missing metadata
+        # - one cell is missing its source
         # - one cell has an invalid level
         with self.fopen(u'invalid.ipynb', u'r') as f:
             nb = read(f, u'json')