Unit tests fail under node 6.10.1

[jfirebaugh]

Unit tests which pass under 6.10.0 are failing on node 6.10.1, released today. The first failing test is:

  1) test/unit/source/canvas_source.test.js CanvasSource rerenders if animated window.useFakeHTMLCanvasGetContext is not a function:
     Error: window.useFakeHTMLCanvasGetContext is not a function
      at createSource (test/unit/source/canvas_source.test.js:11:12)
      at Test.t.test (test/unit/source/canvas_source.test.js:64:24)
      at Test.test (test/unit/source/canvas_source.test.js:63:7)

Failed job: https://circleci.com/gh/mapbox/mapbox-gl-js/7292.

#4470 pins to 6.10.0 temporarily so that we can continue working while this is investigated.

[jfirebaugh]

Release notes: https://nodejs.org/en/blog/release/v6.10.1/

[jfirebaugh]

This was a latent bug exposed by node fixing nodejs/node#6287, which in turn fixes jsdom/jsdom#1622, which we are implicitly relying on due to the order of actions in window.js:

1. Delete own properties in previousWindow (=== module.exports)
2. Extend module.exports with contents of fresh jsdom window
3. Extend fresh jsdom window with our custom properties, e.g. useFakeHTMLCanvasGetContext

(2) and (3) should occur in the opposite order. It happened to work in prior version of node because (1) didn't actually delete our custom properties due to the above linked issues.

PR incoming.

[jfirebaugh]

...turns out fixing that fixes one issue, but there are others. Stay tuned.