Move webpack from post-install to yarn start

Fixes #149.

Procfile

@@ -1 +1 @@
-web: node server.js
+web: yarn start

package.json

@@ -6,8 +6,7 @@
   "scripts": {
     "clean": "rimraf bundle.js bundle.js.map",
     "build": "webpack --config=webpack.config.js -p",
-    "postinstall": "$npm_execpath run clean && $npm_execpath run build",
-    "start": "node server.js",
+    "start": "([ -f .env ] || $npm_execpath run build) && node server.js",
     "dev": "webpack-dev-server --inline --hot",
     "lint": "$npm_execpath run lint:js && $npm_execpath run lint:markdown",
     "lint:js": "eslint --cache --ext .js,.jsx src *.js",

server.js

@@ -10,6 +10,7 @@ app.use(bodyParser.json({ type: 'application/*+json' }));
 app.use(express.static(__dirname));
 
 // Check if we're running on a local dev machine
+// TODO: Remove use of .env. Change instructions to direct users to direnv
 if (fs.existsSync('./.env')) {
   // Load environment variables from .env
   require('dotenv').config();