Upgrade to Django 1.9: Pass key function to `sorted`
Upgrade to Django 1.9: Pass key function to `sorted`

`sorted(word_pairs)` worked, but in Django 1.9 it failed and complained
that it could not compare two word pairs. So now we pass the key
function which tells `sorted` how to compare.

hcs42/ExponWords,hcs42/ExponWords,hcs42/ExponWords,hcs42/ExponWords