wrap extension name in quotes to support uuid-ossp extension

I've manually added the "uuid-ossp" extension by executing raw sql in a migration, but it breaks when running `rake db:schema:load`, with the following error:

```
PG::SyntaxError: ERROR:  syntax error at or near "-"
LINE 1: CREATE EXTENSION IF NOT EXISTS uuid-ossp VERSION '1.0'
```

The extension name needs to be quoted, so this fixes that.