Require net-ssh lower than 2.8.0

Sprinkle SSH authentication does not work with `net-ssh` gem >= 2.8. It simply
fails even if the credentials are correct and were working before the upgrade. 

To reproduce the issue, specify version in Gemfile:

```ruby
gem "net-ssh", "~> 2.8"
gem "sprinkle"
```

Define capistrano deployment:

```ruby
deployment do
  delivery :capistrano do
    set :user,  "user"
    set :password, "password"
    role :app,  "localhost"
  end
```
Requiring `net-ssh` < 2.8 fixed the issue.