Move Rails router code into ae_page_objects-rails

[dtognazzini]

Move all of the Rails specific code out of AePageObjects and into an ae_page_objects-rails gem that declares an explicit dependency on Rails.

Use the most recent version of Rails to run integration tests in ae_page_objects. Test against all supported versions of Rails in ae_page_objects-rails.

[dtognazzini]

#175 moved Rails support to ae_page_objects/rails. Not sure a new gem is warranted. The advantages of moving to a new gem:

1. ae_page_objects becomes free of any Rails code.
2. ae_page_objects tests become simpler as only a single version of Rails would be used for integration tests.
3. ae_page_objects-rails can list explict Rails version dependencies in its gemspec.
4. ae_page_objects-rails and ae_page_objects can evolve independently of each other.

Disadvantages:

1. The default ae_page_objects router (BasicRouter) isn't very useful.
2. AFAICS only Appfolio uses this gem and we mostly use it with Rails projects, so we'd be pulling in both gems in most cases.
3. Yet another gem to maintain