assume valid if there's no schema (v2)

IPython/nbformat/validator.py

@@ -31,6 +31,11 b' def get_validator(version=None):'
 
     if version not in validators:
         v = import_item("IPython.nbformat.v%s" % version)
+        try:
+            v.nbformat_schema
+        except AttributeError:
+            # no validator
+            return None
         schema_path = os.path.join(os.path.dirname(v.__file__), v.nbformat_schema)
         with open(schema_path) as f:
             schema_json = json.load(f)
@@ -98,6 +103,10 b' def validate(nbjson, ref=None, version=None):'
 
     validator = get_validator(version)
 
+    if validator is None:
+        # no validator
+        return
+
     try:
         if ref:
             return validator.validate(nbjson, {'$ref' : '#/definitions/%s' % ref})