Fix event handlers on ruby 2

Fixes #4291.

Because `#respond_to?` no longer returns true for protected methods, the block that actually calls all the event subscribers was dead code on Ruby 2.x

### Verification
 - [ ] Make sure you're using Ruby 2.1.x
 - [ ] Get a session
 - [ ] **Verify** Output contains:
```
[*] Command shell session 1 opened
```