Update Launching Docker section

[noamross]

In chapter 2, the Launching section is a bit out of date, as one no longer needs to use eval "$(docker-machine env default)" or the quickstart terminal with the latest editions of Docker for Mac or Docker for Windows. Instead, you just need to start the Docker application, which runs in the background.

Not sure if anyone is currently maintaining this lesson. It might make sense to add an appropriate http://www.repostatus.org/ badge.