Remove unwanted files + Ruby version

I guess that `log/` and `tmp/` where checkin by accident. I've removed them in this PR and added them to the .gitignore file. 

The `secret_token` will be loaded from the environment variable `SECRET_TOKEN` in production as suggested by [Daniel Fone](http://daniel.fone.net.nz/blog/2013/05/20/a-better-way-to-manage-the-rails-secret-token/).

I hope that I've picked the right Ruby version in the `.ruby-version` file.