Fix for upgrade issue from 0.7.4 to 0.8.0
Fix for upgrade issue from 0.7.4 to 0.8.0

This fixes the following problem found during testing:
```
ALTER EXTENSION cartodb UPDATE TO '0.8.0';
ERROR:  cannot change return type of existing function
HINT:  Use DROP FUNCTION cdb_usertables(text) first.
```

CartoDB/cartodb-postgresql,CartoDB/cartodb-postgresql