use release_path instead of current_path

We define ruby version in Gemfile, also use rvm on production, and recently we upgrade ruby from 2.1.2 to 2.1.3

When deploying with capistrano, it runs `sidekiq:stop` with old `current_path`, Gemfile said 2.1.2 but rvm said 2.1.3, it's fixed by replacing `current_path` with `release_path`