Bugfix: exception if `setup` task is not defined

Support for `setup` task was added so the plugin works better with
`capistrano-unicorn-nginx` and `capistrano-postgresql`.
But, if those plugins aren't used for deployment, `setup` task isn't
defined (capistrano doesn't define it in v3+) so loading the plugin
raises an exception.

Fixes #2
v1.1.1