Moved alias line in order to avoid 'method not found' errors.

Hi! I got this error everytime i tried running middleman

    $ bundle exec middleman
    /.../tumblargh/lib/middleman/extensions/tumblargh.rb:30:in `alias_method': undefined method `find_resource_by_destination_path' for class `Middleman::Sitemap::Store' (NameError)

Moving the alias line to the bottom solved this problem.