Support Ctrl-D to terminate session

pry-remote-em currently blows up if you press Ctrl-D, with:

```
/Users/nbudin/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/pry-remote-em-0.7.5/lib/pry-remote-em/client.rb:250:in `block in readline': undefined method `[]' for nil:NilClass (NoMethodError)
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/eventmachine-1.0.3/lib/eventmachine.rb:951:in `call'
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/eventmachine-1.0.3/lib/eventmachine.rb:951:in `run_deferred_callbacks'
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/eventmachine-1.0.3/lib/eventmachine.rb:187:in `run_machine'
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/eventmachine-1.0.3/lib/eventmachine.rb:187:in `run'
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/pry-remote-em-0.7.5/bin/pry-remote-em:64:in `<top (required)>'
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/bin/pry-remote-em:23:in `load'
	from /Users/nbudin/.rbenv/versions/2.0.0-p195/bin/pry-remote-em:23:in `<main>'
```

This patch simply checks if the line passed to the callback is nil, and if so, assumes the user pressed Ctrl-D to end the session.