Making HMR to work on Windows (#418)

HMR not working on Windows and it took me like 40 minutes to figure it out. I verified this was the cause before sending this PR.

package.json

@@ -9,7 +9,7 @@
     "test-watch": "npm test -- --watch",
     "test-e2e": "cross-env NODE_ENV=test BABEL_DISABLE_CACHE=1 mocha --compilers js:babel-register --require ./test/setup.js ./test/e2e.js",
     "lint": "eslint app test *.js",
-    "hot-server": "node -r babel-register server.js",
+    "hot-server": "cross-env NODE_ENV=development node -r babel-register server.js",
     "build-main": "cross-env NODE_ENV=production node -r babel-register ./node_modules/webpack/bin/webpack --config webpack.config.electron.js --progress --profile --colors",
     "build-renderer": "cross-env NODE_ENV=production node -r babel-register ./node_modules/webpack/bin/webpack --config webpack.config.production.js --progress --profile --colors",
     "build": "npm run build-main && npm run build-renderer",