Fix deployment

Author: c-w

package.json

@@ -6,6 +6,7 @@
   "license": "Apache-2.0",
   "private": true,
   "scripts": {
+    "predeploy": "gh-pages-clean",
     "deploy": "gh-pages --silent --dist src --branch master --repo \"https://${GITHUB_TOKEN}@github.com/${TRAVIS_REPO_SLUG}\"",
     "lint.css": "csslint src",
     "lint.html": "htmllint src/index.html",