Further tweaks to compute_ranks.

[hannosch]

@jaredkerim

I tried to add a couple more tweaks based on your PR, done in an extra commit:

• The country query limits the fields, so it doesn't download additional fields like iso codes, or population estimates.
• The countries are ordered by area in decreasing order instead of name. So largest areas/countries will be checked first, since those have a higher chance of containing the points.
• I've added prepared geometries, which should be faster at contains queries but don't support distance.

I don't have a large enough sample dataset to try this, could you sanity check that this actually improves things?

[coveralls]

Coverage remained the same at 100.0% when pulling 622e06c on hannosch:282-hannosch into 5fa2be8 on mozilla-services:master.

[jaredlockhart]

I'm creating another pull request for this to rebase it on top of master.