Make database adapter selectable for production

Currently Gemfile is optimized for Heroku and installs `pg` gem for the production database adapter. But there may be people who wants to use MySQL instead, so I thought it's better to include both gem as a default and use bundler's `--without` option to skip one of them.
Not sure if this is the best way to solve but it's how [GitLab](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/install/installation.md#install-gems) does.