Add shortcut to delete Travis cache

Useful for when node modules have been deleted

package.json

@@ -25,6 +25,7 @@
     "test:acceptance": "bin/test-acceptance",
     "test:unit": "node test/index.js",
     "test": "npm run lint && npm run test:unit",
+    "travis:deletecache": "travis cache --delete",
     "watch:css": "nodemon --watch src --ext scss -x 'npm run build:css && node website/update.js'",
     "watch:fast": "parallelshell 'npm run watch:css' 'npm run web:preview'",
     "watch:js": "nodemon --watch src --ext js -x 'npm run build:bundle && node website/update.js'",