clean dist before build

package.json

@@ -6,7 +6,7 @@
   "scripts": {
     "dev": "node server",
     "start": "cross-env NODE_ENV=production node server",
-    "build": "npm run build:client && npm run build:server",
+    "build": "rimraf dist && npm run build:client && npm run build:server",
     "build:client": "cross-env NODE_ENV=production webpack --config build/webpack.client.config.js --progress --hide-modules",
     "build:server": "cross-env NODE_ENV=production webpack --config build/webpack.server.config.js --progress --hide-modules"
   },
@@ -37,6 +37,7 @@
     "css-loader": "^0.25.0",
     "extract-text-webpack-plugin": "^2.0.0-beta.3",
     "file-loader": "^0.9.0",
+    "rimraf": "^2.5.4",
     "stylus": "^0.54.5",
     "stylus-loader": "^2.1.2",
     "sw-precache-webpack-plugin": "^0.5.1",

yarn.lock

@@ -2925,7 +2925,7 @@ right-align@^0.1.1:
   dependencies:
     align-text "^0.1.1"
 
-rimraf@^2.2.8, rimraf@~2.5.1, rimraf@~2.5.4, rimraf@2:
+rimraf, rimraf@^2.2.8, rimraf@~2.5.1, rimraf@~2.5.4, rimraf@2:
   version "2.5.4"
   resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-2.5.4.tgz#96800093cbf1a0c86bd95b4625467535c29dfa04"
   dependencies: