Require delegate since Ruby 2.7 does not auto-include this

Previous RubyGems versions used to load delegate indirectly, but
this has changed with Ruby 2.7:
https://stackoverflow.com/a/60468620/1992201

lib/rspec_flaky/flaky_examples_collection.rb

 # frozen_string_literal: true
 
 require 'active_support/hash_with_indifferent_access'
+require 'delegate'
 
 require_relative 'flaky_example'