Running SSHKit tests with Rake 11.0 reveals many Ruby warnings

Rake 11.0 turns on Ruby warnings by default. With warnings enabled, running SSHKit's test suite results in a flood of messages like this:

```
lib/sshkit/runners/group.rb:15: warning: instance variable @group_size not initialized
lib/sshkit/host.rb:59: warning: instance variable @local not initialized
lib/sshkit/backends/abstract.rb:141: warning: instance variable @pwd not initialized
lib/sshkit/backends/abstract.rb:149: warning: instance variable @env not initialized
lib/sshkit/backends/abstract.rb:149: warning: instance variable @user not initialized
lib/sshkit/backends/abstract.rb:149: warning: instance variable @group not initialized
```

We could:
- Treat these warnings as legitimate and try to fix them; or
- Turn off Ruby warnings

I haven't had time to think this through yet, so for now I'm just opening it for discussion.

Your thoughts?