I think that we should check if ```/Library/RubyMotion.old``` already exists and print a message here if it's the case (and quit).

Also, I'm wondering if it wouldn't be better to move ```/Library/RubyMotion``` into a temporary + unique directory, to avoid any issue. 

Untested:

```
require 'tmpdir'
backup_dir = File.join(Dir.mktmpdir, 'Library')
...
FileUtils.mv '/Library/RubyMotion', backup_dir
...
if File.exist?(backup_dir)
  FileUtils.mv 'backup_dir', '/Library/RubyMotion'
end
```