Ensure Slate enabled for assistive devices

This actually didn't work since the `install` script ran after the
`setup` script, so the `cask_installed` check would always return true,
causing the code not to run.

Instead, make sure we're on OS X and then just always run the SQL insert
command.
Add Shortcat plugin

Shortcat makes it easier to work without a mouse. Install it and enable
assistive devices automatically.