Add phase_order rake task and update README

[kkohrt]

The most confusing part about the proposed dependency graph configuration was understanding what the default order would be. And the biggest risks seemed to be in accidentally creating a circular dependency or specifying a dependency incorrectly. This change addresses some of those issues as follows"

1. Updates the phases class to coerce phase names into symbols, and dependencies into arrays of symbols
2. Introduces a rake task that prints out the order of phases and their dependencies.

It also does a major overhaul to the README that reflects the additional information and insights I gained while working in the code and testing the dependency features that I wish would have been in the README to begin with. Also: Updates the example rake tasks to a more modern Rails format, and the Procore logo to use the procore-oss source.

Output of new rake task (when run from /spec/dummy test application):

$ bundle exec rake handcuffs:phase_order

Configured Handcuffs phases, in order, are:
  1. pre_restart, requires: (nothing)
  2. post_restart, requires: pre_restart
  3. data_migrations, requires: pre_restart

I get the same output when I "mangle" the configuration as follows

  config.phases = {
    # Prevent running post_restart migrations if there are outstanding
    # pre_restart migrations
    post_restart: :pre_restart,
    # Require pre_restarts before data_migrations, but do not enforce ordering
    # between data_migrations and post_restarts
    data_migrations: 'pre_restart',
    # pre_restarts have no prerequisite phases
    'pre_restart' => []
  }

Checklist:

• I have updated the necessary documentation
• I have signed off all my commits as required by DCO
• My build is green