Include rake to avoid rails server to crash on bootstrap + move triggers from code to deploy.rb of the project

I had problem starting my server with the current implementation.

`lib/slackistrano/tasks/slack.rake:2:in `<top (required)>': undefined method `namespace' for main:Object (NoMethodError)`

It apprends that `require 'rake'` was needed.

But then the hooks `after 'deploy:starting', 'slack:deploy:starting'` were not accepted anymore:

`<top (required)>': undefined method after' for main:Object (NoMethodError)`

So I decided to removed the trigger from the configuration  `set :slack_run_starting, ->{ true }` and propose to had the hooks manually inside the deploy.rb like the other gems are doing.

It works fine for me, so I propose the pull request. However, feel free not to merge it, I solve my own problem and it doesn't mean that it's the best way for everyone using it.