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.
Merge branch 'patch-1' of https://github.com/dmarkow/capistrano into dmarkow-patch-1

Conflicts:
	CHANGELOG.md