Don't blow up on a MemoryError

My VM that re-generates the stats is now breaking on a MemoryError when trying
load the local cache.  Ignore this for now since it will fall back to querying
gerrit.

Change-Id: I151fae89d9fb9a45d72e72c5aade4cb3342b4267

reviewstats/utils.py

@@ -81,7 +81,10 @@ def get_changes(projects, ssh_user, ssh_key, only_open=False, stable='',
                 mtime = os.stat(pickle_fn).st_mtime
                 if (time.time() - mtime) <= CACHE_AGE:
                     with open(pickle_fn, 'r') as f:
-                        changes = pickle.load(f)
+                        try:
+                            changes = pickle.load(f)
+                        except MemoryError:
+                            changes = None
 
         if not changes:
             while True: