[Capistrano] added update seed data task

Added update seed data capistrano task.

For deploy with seed data update use following:
  1. Add rake task to lib/tasks/update_seed_data.rake (db:update_seed_data)
  2. Add to config/deploy/<env>rb following:
```ruby
  if ENV["UPDATE_SEED"]
    after "deploy:finishing", "deploy:update_seed_data"
  end
```
  3.Then you can run 'cap deploy' with seed data update

```ruby
  cap staging deploy UPDATE_SEED=true
```