Allow absolute pid paths (for Capistrano)

The rake tasks forces `sidekiq_pid` to be relative to `Rails.root`.

```ruby
    # must be relative to Rails.root. If this changes, you'll need to manually
    # stop the existing sidekiq process.
    set :sidekiq_pid,             ->{ "tmp/sidekiq.pid" }
```

This causes an issue when deploying with Capistrano: some tasks are executed before the symlinks are created, thus causing the file test (`[-f path_to_pid ]`) to fail when it shouldn't.

I will submit a fix shortly.

PS: Awesome gem. 1,000 open source points to the maintainer(s)!