Set cache pickle protocol to 2.

HIGHEST_PROTOCOL will cause it to change when we move from python 2 to 3
and this may cause issues as older machines try to read objects pickeld
with a version of the protocol they don't know.

During deploys, and if we have to roll back are two examples.

openedx/core/lib/cache_utils.py

@@ -151,7 +151,7 @@ class process_cached(object):  # pylint: disable=invalid-name
 
 def zpickle(data):
     """Given any data structure, returns a zlib compressed pickled serialization."""
-    return zlib.compress(pickle.dumps(data, pickle.HIGHEST_PROTOCOL))
+    return zlib.compress(pickle.dumps(data, 2))  # Keep this constant as we upgrade from python 2 to 3.
 
 
 def zunpickle(zdata):