Fix schema dump for postgres

I had trouble getting enum_tables to work with postgres. Specifically, the schema dumping would not work correctly.

I tracked the bug down to the use of the ActiveRecord "execute" method. As far as I know, that method is not portable across databases, since different database adapters may return the results in different formats.
Using #select_rows and #select_values should fix this. I have tested that this works (at least in my case) with both MySQL and Postgres.