Fix issue where rev detection fails with additional JSON files

[jacobwgillespie]

I have a copy task that copies over additional supporting files which include a manifest.json that is necessary for add to homescreen on Android. Currently copying this JSON file fails with gulp-rev-collector because it tries to take path.basename of an object. This PR adds a check to make sure the value is a string, fixing that error.

For reference, this is the JSON file that breaks the package (since it tries to take path.basename of the icons array):

{
  "name": "Half Staff",
  "icons": [
    {
      "src": "/images/icons/android-chrome-36x36.png",
      "sizes": "36x36",
      "type": "image/png",
      "density": 0.75
    },
    {
      "src": "/images/icons/android-chrome-48x48.png",
      "sizes": "48x48",
      "type": "image/png",
      "density": 1
    },
    {
      "src": "/images/icons/android-chrome-72x72.png",
      "sizes": "72x72",
      "type": "image/png",
      "density": 1.5
    },
    {
      "src": "/images/icons/android-chrome-96x96.png",
      "sizes": "96x96",
      "type": "image/png",
      "density": 2
    },
    {
      "src": "/images/icons/android-chrome-144x144.png",
      "sizes": "144x144",
      "type": "image/png",
      "density": 3
    },
    {
      "src": "/images/icons/android-chrome-192x192.png",
      "sizes": "192x192",
      "type": "image/png",
      "density": 4
    }
  ],
  "start_url": "https://halfstaff.co",
  "display": "standalone"
}