Update Middleman to version 3.1.1; bundle update.
Change Google Analytics tracking gem.

https://github.com/leehambley/rack-google-analytics
Remove Discount markdown engine.

Middleman 3.1 now uses the Kramdown markdown engine by default.
Disable printing of GitHub banner.
MySQL: group post-installation steps together.
Update versions.
Pow: add links and dependencies; mention upgrade.