Add Node >= 4 requirement to package.json and packager/package.json

Summary: When you try to install these with old Node you'll now get a message from npm telling you that your version of Node is old. This makes it more obvious what's going on and hopefully reduces the number of issues we get due to people using an old version of Node.
Closes https://github.com/facebook/react-native/pull/3296

Reviewed By: @​svcscm

Differential Revision: D2526500

Pulled By: @vjeux

fb-gh-sync-id: 036e10a8d1819ea082e419cd328a458202f0e071

package.json

@@ -7,6 +7,9 @@
     "type": "git",
     "url": "git@github.com:facebook/react-native.git"
   },
+  "engines": {
+    "node": ">=4"
+  },
   "jest": {
     "scriptPreprocessor": "jestSupport/preprocessor.js",
     "setupEnvScriptFile": "jestSupport/env.js",

packager/package.json

@@ -6,6 +6,9 @@
     "type": "git",
     "url": "git@github.com:facebook/react-native.git"
   },
+  "engines": {
+    "node": ">=4"
+  },
   "jest": {
     "setupEnvScriptFile": "jestSupport/env.js",
     "testPathIgnorePatterns": [