Simplify shutdown logic for Connection

* Use a QSocketNotifier listening to stdin to determine when the parent
  process has detached.
Simplify Travis builds

* Don't test on 1.9.3 anymore. 2.1.2 should be sufficient.
* Don't test Capybara 2.0 compatibility. We'll be adding 2.3 support
  soon and the build matrix is already complicated enough.