Use pure XBlock instead XModule.

peer_grading/xblock/peer_grading_block.py

+import logging
+
+from xblock.core import XBlock
+
+log = logging.getLogger(__name__)
+
+
+class PeerGradingBlock(XBlock):
+    pass
\ No newline at end of file

peer_grading/xblock/peer_grading_module.py

-import logging
-from x_module import XBlock
-from xmodule.raw_module import RawDescriptor
-
-log = logging.getLogger(__name__)
-
-
-class PeerGradingFields(object):
-    """
-    @todo We'll need some Peer Grading Fields to define what information we want to pass to the grading workflow.
-    """
-    pass
-
-
-class PeerGradingModule(PeerGradingFields, XBlock):
-
-    _VERSION = 0
-
-
-class PeerGradingDescriptor(PeerGradingFields, RawDescriptor):
-    """
-    Module for adding peer grading questions
-    """
-    # TODO What is a descriptor?
-    pass

requirements/base.txt

 # edX Internal Requirements
+-e git+https://github.com/edx/XBlock.git@fa88607#egg=XBlock
 
 # Third Party Requirements
 django==1.4.8