[experiments][models][block_input] Added dependencies to BlockInput natural_key

This allows Django to serialize the database content in the correct
order.

beat/web/experiments/models/block_input.py

@@ -81,3 +81,6 @@ class BlockInput(models.Model):
         cache_hash = self.cache and self.cache.hash
         database_natural_key = self.database and self.database.natural_key()
         return (block_natural_key, cache_hash, database_natural_key)
+    natural_key.dependencies = ['experiments.block',
+                                'experiments.cachedfile',
+                                'databases.databasesetoutput']