Update readme to include fixes for rails 3.2.x.

Rails ignores the database.yml when DATABASE_URL is defined, but rails 3.2.x does not correctly parse `prepared_statements=false`.

prepared_statements=false is evaluated to `{prepared_statements: "false"}`, rather than `{prepared_statements: false}`. The string `"false"` evaluates to true, so prepared statements are not disabled correctly.

The monkey patch filters the config options hash, and correctly maps the `"false"` to `false`.