Specify ruby version 1.9.3 in Gemfile

As ruby 1.9 is [required for kandan](https://github.com/kandanapp/kandan/blob/master/DEPLOY.md#requirements).
Specifying it in Gemfile is requiered for Heroku deployment [since ruby 2 is default](https://blog.heroku.com/archives/2013/6/17/ruby-2-default-new-aps).