use docker for development

see [README](https://github.com/bitzesty/content-planner/blob/ab7222bdc1aa15acef38abd829a5dc543bae3287/README.md#development-via-vagrant-and-docker)
I couldn't get ruby 2.0.0 to compile on ubuntu 14.04. According to [this question](http://stackoverflow.com/questions/23155289/rbenv-build-failed-on-ubuntu-14-04) it's a bug.