Set up spring

This allows us to do Rails things in development with less start-up
time. For example, to start the Rails server now, you can execute:

  bin/rails s

https://github.com/rails/spring
Add Procfile.dev

This allows us to start up all of the processes needed to run Diffux in
development with a single command (after `gem install foreman`):

  foreman start -f Procfile.dev