Use a string for the ensure_stage task

Rake 10.2.0 introduced new task name checking which started raising `TypeError: no implicit conversion of Rake::Task into String` errors when trying to deploy. This is because `@top_level_tasks` is expected to contain strings, not Rake::Task objects.

CHANGELOG.md

@@ -7,6 +7,7 @@ Reverse Chronological Order:
 * Minor changes:
   * Added `keys` method to Server properties to allow introspection of automatically added
     properties.
+  * Compatibility with Rake 10.2.0 - `ensure_task` is now added to `@top_level_tasks` as a string. (@dmarkow)
 
 ## `3.1.0`
 

lib/capistrano/application.rb

@@ -24,7 +24,7 @@ module Capistrano
       if tasks_without_stage_dependency.include?(@top_level_tasks.first)
         @top_level_tasks
       else
-        @top_level_tasks.unshift(ensure_stage)
+        @top_level_tasks.unshift(ensure_stage.to_s)
       end
     end