`airplane-mode` should also prevent update checks

[d4mation]

The airplane-mode feature is intended to prevent WP from making external calls, but it would be nice if that setting also prevented slic itself from making external calls.

GitHub was down a bit today and it resulted in slic hanging as it attempted to check for updates before each run.

This can be reproduced by disconnecting from the Internet on your machine and seeing how it takes substantially longer to begin running tests while it tries to reach out to GitHub. In my case, with an Internet connection my limited set of tests took 10s to run and without an Internet connection it took 17s.

Another way to reproduce this, which seems to emulate what I was experiencing while GitHub was down more closely, would be to adjust your hosts file so that github.com points to an invalid, remote IP. No matter how long I wait, it never proceeds to run the tests as it is stuck trying to reach out to check for updates. Once the adjustment is removed, slic immediately finds GitHub and continues to run the tests.

Allowing the ability to disable this update check could be useful in the event that GitHub were to go down similarly in the future, or even just if the user doesn't have an active internet connection at the time.