Made React version not exact again

Summary:
React is a peer dependency and apps may depend on a wider range of versions of React, so strict dependency is not a good choice.
Also `react-native init` does `npm install react` internally creating package.json with `react: ^0.15.x`, this change makes versions consistent as well.

Reviewed By: matryoshcow

Differential Revision: D3365433

fbshipit-source-id: d2810662c36129ff9af184c359ac190544db75da

package.json

@@ -124,7 +124,7 @@
     "react-native": "local-cli/wrong-react-native.js"
   },
   "peerDependencies": {
-    "react": "15.1.0"
+    "react": "^15.1.0"
   },
   "dependencies": {
     "absolute-path": "^0.0.0",
@@ -191,7 +191,7 @@
     "flow-bin": "^0.26.0",
     "jest": "12.1.1",
     "portfinder": "0.4.0",
-    "react": "15.1.0",
+    "react": "^15.1.0",
     "shelljs": "0.6.0"
   }
 }