Fix killing webkit_server on JRuby

The connection now waits until `webkit_server` has started so that it can discover the pid.

We have not added any additional tests as the current suite has plenty of failures which capture this problem. We have not fixed the rest of the failing tests on JRuby, but at least with this patch you should no longer so the following error.

```
TypeError: no implicit conversion from nil to integer
                    kill at org/jruby/RubyProcess.java:971
                    kill at org/jruby/RubyProcess.java:886
            kill_process at /Users/pivotal/workspace/capybara-webkit/lib/capybara/webkit/connection.rb:73
  register_shutdown_hook at /Users/pivotal/workspace/capybara-webkit/lib/capybara/webkit/connection.rb:64
```