Add app.json, mod client/package.json for node-sass deploy (shakacode#389)

* Add app.json and modify client/package.json

Because of Node-sass issues with yarn
Per yarnpkg/yarn#1981 (comment)

Author: justin808

app.json

@@ -0,0 +1,5 @@
+{
+  "env": {
+    "NODE_MODULES_CACHE": "false",
+  }
+}

client/package.json

@@ -25,6 +25,7 @@
   },
   "homepage": "https://github.com/shakacode/react-webpack-rails-tutorial",
   "scripts": {
+    "postinstall": "npm rebuild node-sass",
     "test": "NODE_PATH=./app mocha --compilers js:babel-core/register --require ./app/libs/testHelper.js --require ./app/libs/testNullCompiler.js 'app/**/*.spec.@(js|jsx)'",
     "test:debug": "yarn run test -- --debug-brk",
     "start": "babel-node server-express.js",