Update tpl-loader to webpack 4

package.json

@@ -124,7 +124,7 @@
     "stylelint": "^8.2.0",
     "stylelint-config-property-sort-order-smacss": "^2.0.0",
     "stylelint-config-standard": "^17.0.0",
-    "tpl-loader": "CartoDB/tpl-loader#master",
+    "tpl-loader": "CartoDB/tpl-loader#webpack4",
     "uglify-js": "2.7.x",
     "uglifyjs-webpack-plugin": "https://registry.npmjs.org/uglifyjs-webpack-plugin/-/uglifyjs-webpack-plugin-1.2.5.tgz",
     "watchify": "3.7.0",