Only define flexbox for media:screen. Fixes print cropping bug in FF.
Merge branches 'master' and 'master' of github.com:andru/booty

* 'master' of github.com:andru/booty:
  Append and prepend to footer zones.
  Move navbar to 'before:lower-container' zone.

* 'master' of github.com:andru/booty:
  Append and prepend to footer zones.
  Move navbar to 'before:lower-container' zone.