feat(build): Add an integration test that runs on Travis

This test, after performing the usual Nixery build, loads the built
image into Docker, runs it, pulls an image from Nixery and runs that
image.

To make this work, there is some configuration on the Travis side.
Most importantly, the following environment variables have special
values:

* `GOOGLE_KEY`: This is set to a base64-encoded service account key to
  be used in the test.
* `GCS_SIGNING_PEM`: This is set to a base64-encoded signing key (in
  PEM) that is used for signing URLs.

Both of these are available to all branches in the Nixery repository.

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