[Rails5] Update `ApplicationController#log_exception` to fix `undefined method 'clean'` error

This commit fixes the error:
```
  1) Projects::TodosController Merge Requests POST create when not authorized for merge_request doesn't create todo
     Failure/Error: application_trace = ActionDispatch::ExceptionWrapper.new(env, exception).application_trace

     NoMethodError:
       undefined method `clean' for #<Hash:0x000055be5bda35d0>
       Did you mean?  clear
     # ./app/controllers/application_controller.rb:113:in `log_exception'
     # ./app/controllers/application_controller.rb:40:in `block in <class:ApplicationController>'
     # ./spec/controllers/projects/todos_controller_spec.rb:80:in `go'
     # ./spec/controllers/projects/todos_controller_spec.rb:138:in `block (6 levels) in <top (required)>'
     # ./spec/controllers/projects/todos_controller_spec.rb:138:in `block (5 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # ActiveRecord::RecordNotFound:
     #   Couldn't find MergeRequest
     #   ./app/finders/concerns/finder_methods.rb:19:in `raise_not_found_unless_authorized'

Finished in 7.53 seconds (files took 12.8 seconds to load)
1 example, 1 failure
```

Also see https://github.com/rails/rails/commit/6d85804bc6aeecce5669fb4b0d7b33c069deff3a

app/controllers/application_controller.rb

@@ -110,7 +110,8 @@ class ApplicationController < ActionController::Base
   def log_exception(exception)
     Raven.capture_exception(exception) if sentry_enabled?
 
-    application_trace = ActionDispatch::ExceptionWrapper.new(env, exception).application_trace
+    backtrace_cleaner = Gitlab.rails5? ? env["action_dispatch.backtrace_cleaner"] : env
+    application_trace = ActionDispatch::ExceptionWrapper.new(backtrace_cleaner, exception).application_trace
     application_trace.map! { |t| "  #{t}\n" }
     logger.error "\n#{exception.class.name} (#{exception.message}):\n#{application_trace.join}"
   end