update database to be .db which is what I normally do nowadays

and pin developers to django 1.7 so the 'migrate' command is available
refactor render_change_form context to map for readability

to me, it makes more sense to read map() instead of a list comprehension
add some more docs here in there as I try and remember what I wrote
add a tiny bit more test coverage
Merge pull request #28 from texastribune/random-tweaks

Random tweaks