[ci skip] Add missing dom5 devDependency

This *just worked* with npm 3 because of flattened tree from
gulp-vulcanize, but dependencies should be explicit

Author: dfreedm

package.json

@@ -9,6 +9,7 @@
   },
   "devDependencies": {
     "del": "^2.2.0",
+    "dom5": "^1.3.1",
     "eslint-plugin-html": "^1.3.0",
     "gulp": "^3.8.11",
     "gulp-audit": "^1.0.0",