Refactor setup script

- `yarn` (install) required prior executing scripts from fresh clone
  - documented in ReadMe
- remove redundant `yarn` (install) within setup script
- remove redundant `yarn` before executing <command>
  - resolves to executable in `node_modules/.bin`

Author: leotm

package.json

@@ -3,7 +3,7 @@
   "version": "0.0.1",
   "private": true,
   "scripts": {
-    "setup": "yarn && yarn allow-scripts",
+    "setup": "allow-scripts",
     "start": "react-native start",
     "start:reset": "react-native start --reset-cache",
     "android": "react-native run-android --active-arch-only",