Broken emoji link

While reviewing #513, I noticed the Travis failure was pointing to a broken link:

./output/hacking-atom/sections/package-word-count/index.html

External link https://a248.e.akamai.net/assets.github.com/images/icons/emoji/unicode/1f600.png failed: 404 No error

I think we might need to update

flight-manual.atom.io/nanoc.yaml

Line 10 in f7c5d3c

asset_root: https://a248.e.akamai.net/assets.github.com/images/icons

and:

flight-manual.atom.io/nanoc.yaml

Line 21 in f7c5d3c

asset_root: https://a248.e.akamai.net/assets.github.com/images/icons

to use https://github.githubassets.com/images/icons?

ref: jekyll/jemoji#89