Update legacy comment
Merge pull request #3008 from weih/update-legacy-comment

Update legacy comment [ci skip]
Start adding code to allow devise_for to specify the engine context (router_name) to be used.
Make after_sign_out_path_for and after_inactive_sign_up_path_for aware of router name if declared in devise_for declaration.
Fix bug in the tests
Get the logic in the right order and fix variable naming (and run the tests)
Add find_mapping! method to return the router_name instead of overloading find_scope!
make the find_mapping! method return a mapping object
Don't couple the routing object to the whole system.. pass around the RoutingDetails object instead.

NOTE: RoutingDetails is supposed to be a simple data transfer object.
Add description around the new method.
Extract routing context to its own file.
Make sure I include the new class into the gem
Use 2.1 instead of 2.1.0
travis-ci/travis-ci#2220
Update main Gemfile dependencies.
Update 3-2-stable Gemfile dependencies.
Update 4-0-stable Gemfile dependencies.
Update Rails HEAD Gemfile dependencies.
Update Travis config to test on ruby 2.1.1 instead of 2.1.0
split send_reset_password_instructions into two parts for better hooking
rename method + remove extra db call