Run Heroku plugin task to set up for Heroku in production.

I'm not sure if it's okay to have overwritten the rakefile entirely,
the lines it dropped might be useful:

-require 'bundler/setup'
+require File.dirname(__FILE__) + '/config/boot.rb'
+require 'thor'
 require 'padrino-core/cli/rake'

-PadrinoTasks.use(:database)
-PadrinoTasks.use(:datamapper)
 PadrinoTasks.init