Sorts asset map paths to replace by length descending.

This addresses the issue from https://github.com/rickharrison/broccoli-asset-rev/issues/28

If a file has the same name, but with 2 different extensions, it is possible that the shorter extension will replace the longer one. I.E. a file with the extension `woff` will replace the source of `woff2`. To fix this, the plugin will now sort by the asset map length so that longer extensions are always replaced first.