fix params for kextunload

use proper kextunload params
:kext in the cask realm is the bundle identifier (or bundle-id) and should be passed as `-b bundle-id`

to reproduce:
```
$ cask install karabiner
> run karabiner from spotlight and then quit (this loads/activates the kext)
$ cask uninstall karabiner
==> Removing Binary symlink: '/usr/local/bin/warp-mouse-cursor-position'
==> Removing Binary symlink: '/usr/local/bin/blueutil'
==> Running uninstall process for karabiner; your password may be necessary
==> Quitting application ID org.pqrs.Karabiner
==> Unloading kernel extension org.pqrs.driver.Karabiner
==> Can't create org.pqrs.driver.Karabiner.
==> (kernel) Kext -- not found for unload request.
==> Failed to unload -- - (libkern/kext) not found.
Error: Command failed to execute!

==> Failed command:
["/usr/bin/sudo", "-E", "--", "/sbin/kextunload", "-b", "--", "org.pqrs.driver.Karabiner"]

==> Output of failed command:


==> Exit status of failed command:
#<Process::Status: pid 97275 exit 1>
```