Return feature.id in response

[springmeyer]

This:

• Starts returning the feature.id in the response per
• Adds a new fixture for coverage of non-zero feature ids
• Adds process.env.UPDATE to make it easy to update the fixtures in the future (if needed)

If a Feature has a commonly used identifier, that identifier SHOULD be included as a member of the Feature object with the name "id", and the value of this member is either a JSON string or number.

From https://tools.ietf.org/html/rfc7946#section-3.2

/cc @mapbox/core-tech

[springmeyer]

Looks like tests are all failing on linux and only passing on OS X (https://travis-ci.org/mapbox/vtquery/builds/395219301). Sorting is probably the culprit, but not sure yet. Will need to re-read #44 to see if that could be related.

[springmeyer]

Using std::stable_sort to debug this over in e518d71.

[springmeyer]

Stable sort changed results locally on OS X, but looks like it means the results are stable between OS X and Linux now: https://travis-ci.org/mapbox/vtquery/builds/395225756