Explicitly require `active_support/inflector` when using `#tableize`

The `#create_workflow_diagram` method uses the `#tableize` method, which is a member of the
[`ActiveSupport#Inflector`](http://api.rubyonrails.org/classes/ActiveSupport/Inflector.html#method-i-tableize) library.

If we are going to continue to use it, we need to include the `active_support` lib explicitly for non-Rails users.
Otherwise, we'll get a `NoMethodError` when attempting to run.