Update `subliminal-test` to take advantage of `instruments` 5.1.

Resolves #143--we now use `instruments` 5.1's ability to select the device type and SDK, and ask the user to pre-authorize the test machine rather than provide their login password to `subliminal-test`.

[The wiki](https://github.com/inkling/Subliminal/wiki/Continuous-Integration) has already been updated in anticipation of these changes.