Remove Spork and Guard

Use Zeus or Commands instead:

https://github.com/burke/zeus
https://github.com/rails/commands

They are superior to Spork for the following reasons:

* You don't have to commit anything into your Gemfile, spec/spec_helper,
  or version control.
* They improve the Rails bootup speed of rails server, runner, generate,
  console, and rake tasks... not just tests.