Simpler access to state names of AASM

[GMorris-professional]

A small PR to remove the need to call map(&:name) to get the names of the states.

With the following example state machine:

class SimpleExample < ActiveRecord::Base
  include AASM

  aasm do
    state :sleeping, initial: true
    state :running
    state :cleaning
  end
end

example = SimpleExample.new
example.aasm.state_names #[:sleeping, :running, :cleaning]

[codeclimate]

Code Climate has analyzed commit 47a6265 and detected 0 issues on this pull request.

View more on Code Climate.