allow the clean-branches executable to be symlinked somewhere else

This allows the `clean-branches.rb` file to be symlinked from somewhere else. 

For example, I have the `clean-branches` repository in `~/isaac/github/clean-branches`.

However, I want it to be available on my path, so I created a symlink in `/usr/local/bin`:

`/usr/local/bin/clean-branches -> ~/isaac/github/clean-branches`.

However, without this modification, the script errors because it cannot find `lib/cleaner/` (as it is looking for it relative to the current directory: `/usr/local/bin/lib/cleaner`).

Let me know if you have any questions.