Added deploy:set_rails_env to run after invoking stage

Hello! 

What about setting RAILS_ENV before all tasks after invoking stage?

```ruby
Capistrano::DSL.stages.each do |stage|
  after stage, 'deploy:set_rails_env'
end
```

It'l be useful in other gems using RAILS_ENV for transparent support, ex. capistrano3-rails etc.