switch to diskutil for eject

Mostly to see if this reduces random Travis errors, but also
because `diskutil eject` is a documented interface, and
`hdiutil eject` is not.

lib/cask/container/dmg.rb

@@ -45,7 +45,7 @@ def assert_mounts_found
   def eject!
     @mounts.each do |mount|
       # realpath is a failsafe against unusual filenames
-      @command.run!('/usr/bin/hdiutil', :args => ['eject', Pathname.new(mount).realpath])
+      @command.run!('/usr/sbin/diskutil', :args => ['eject', Pathname.new(mount).realpath])
     end
   end
 end