Update readme to include fixes for rails 3.2.x.

We hit this issue while trying to use the buildpack with a rails 3.2 app. No matter what we did, in the heroku environment prepared statements were still being generated, and our staging server became a mountain of error messages.

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`.