Split task :update_symlinks on two: :update_symlinks and :set_current_release

Splitting the task into two parts, will able to perform additional actions
in the latest release path before it becomes the current path.
So that will make deployment more flexible.

As an example, this approach allows to perform pre-compilation of assets in the latest release path instead of the current path.
And as result the ability to make zero-downtime deployment.