Fix for prepared statement already exists (Postgres)

This PR should fix https://github.com/rails/rails/issues/1627, explanation: https://github.com/rails/rails/issues/1627#issuecomment-62866211
The newest entry for the CHANGELOG should go to the top. In addition, please add a new line for the issue that is being fixed to ensure that the format is consistent with the other [entries](https://github.com/abezzub/rails/blob/postgres-bug/activerecord/CHANGELOG.md). Thanks :smile: