Upgrade nginx on servers for security fix (CVE-2017-7529)

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A security update for nginx was released yesterday. While hopefully this shouldn't really affect us since we don't use nginx's cache, it sounds like it's theoretically possible 3rd party modules might be affected (and we do have some 3rd party modules). So in any case, it would be good for us to update.

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Everything should be upgraded now:

• The nginx instance used for our automatic SSL was upgraded earlier today: https://github.com/18F/api.data.gov-ops/commit/6af69ea1c662bfad71905ae9407f32111de3e304
• The nginx instance inside API Umbrella was just upgraded as part of API Umbrella's v0.14.3 package release: https://github.com/18F/api.data.gov-ops/commit/694bd7490f82a41d078060483e2c92deac10ebcd