Fix reference to this in _maybeUnloadDoc inner function

[ericyhwang]

The inner function was introduced in introduced in a320772 (PR #276).

I missed changing a this to model.

I'll look into adding a test later, I want to get this fix out quickly.

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This can cause an error like TypeError: Cannot read property '$remote' of undefined in listeners that are checking properties the pass object.

Stack trace will have a snippet like this:

  at Model.emit [as _emit] (events.js:203:15)
  at Model.emit (.../node_modules/racer/lib/Model/events.js:99:10)
  at unloadDoc (.../node_modules/racer/lib/Model/subscriptions.js:197:11)
  at Model._maybeUnloadDoc (.../node_modules/racer/lib/Model/subscriptions.js:186:5)
  at finishUnfetchDoc (.../node_modules/racer/lib/Model/subscriptions.js:127:11)
  at Model.unfetchDoc (.../node_modules/racer/lib/Model/subscriptions.js:122:5)
  at Model._forSubscribable (.../node_modules/racer/lib/Model/subscriptions.js:66:24)
  at Model.unfetch (.../node_modules/racer/lib/Model/subscriptions.js:21:8)

[ericyhwang]

Fix published in https://github.com/derbyjs/racer/releases/tag/v0.9.13