Invalidate checks cache on translation update

We might be more clever on this and do the cleanup conditionally, but
for now this is safe solution as the checks might become stale.

This completes fix for issue #233

trans/models/translation.py

@@ -535,6 +535,10 @@ class Translation(models.Model):
         # Update revision and stats
         self.update_stats()
 
+        # Cleanup checks cache if there were some deleted units
+        if len(deleted_checksums) > 0:
+            self.invalidate_cache()
+
         # Store change entry
         if request is None:
             user = None