Directories ending in `.js` break

This is the patch for [issue 14](https://github.com/mmun/broccoli-es6-module-transpiler/issues/14) along with a test case that shows the error occurring when the directory check comes after the check for `path.extname(relPath) === '.js'`.
Can you add to this comment to indicate that `path.extname` removes the `/` added by `walkSync`?