Fix boolean checks in `subliminal-test`.

$ENABLE will evaluate to true if ENABLE is not set.
`subliminal-test` always logs that it is preparing to install the app.

Whether in the simulator _or_ on the device.
Enforce that `subliminal-test` will always use the latest simulator if no SDK is specified.

`instruments`' behavior varies across SDK versions, as described in
http://openradar.appspot.com/radar?id=3107401, but we had previously
documented that `subliminal-test` would use the latest version if no SDK
was specified.

We no longer mention modifying the `/etc/authorization` workaround
for authorizing instruments, because even if that worked, it wouldn't be
sufficient to allow the user to avoid specifying their login password or using
the `--live` option: we need authorization to enforce a consistent SDK.
Adjust a device type for Xcode 5.x.
Merge the various simulator scripts into `simulator-set`.

To eliminate redundant logic and simulator restarts, and simplify the CI setup.
When resetting the simulator, remove all support directories for a given SDK version.
Remove `enable_GUI_scripting.scpt`.

Its logic has been incorporated into `authorize_instruments.scpt`,
which was the only script which needed it anymore.
Merge pull request #117 from inkling/jeff/CI_cleanup

CI fixes and refactoring.