fix conferences routes to allow GETs again

for now

Change-Id: I6db47d461961bfc83a8d3d1e6f5c060616879c16
Reviewed-on: https://gerrit.instructure.com/44137
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

config/routes.rb

@@ -158,8 +158,8 @@ CanvasRails::Application.routes.draw do
 
   concern :conferences do
     resources :conferences do
-      post :join
-      post :close
+      match :join, via: [:get, :post]
+      match :close, via: [:get, :post]
       get :settings
     end
   end