making sure that path.normalize gets strings

I've ran in to weird problems on a Linux box using a deprecated NodeJS version.

Problem was:
Warning: Object dist/index.html has no method 'charAt' Use --force to continue.
TypeError: Object dist/index.html has no method 'charAt'
  at Object.exports.normalize (path.js:336:27)
  at isPathEqual (/export/home/WWW/ci/workspace/BFS-T01-build_test/node_modules/grunt-cache-invalidate/tasks/cache_invalidate.js:21:44)