Drop objects in reverse order in downgrade script
Drop objects in reverse order in downgrade script

Otherwise it fails downgrading, despite it is supposed to run everything
in the context of a transaction:

```
tests=# alter extension crankshaft update to '0.0.4';
ERROR:  cannot drop function cdb_pyaggs(numeric[],numeric[]) because
other objects depend on it
DETAIL:  extension crankshaft depends on function
cdb_pyaggs(numeric[],numeric[])
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
```

CartoDB/crankshaft,CartoDB/crankshaft