Made react installed via reac-native init strict to unbreak 15.1.0

Summary:
[email protected] is incompatible with [email protected].
This PR was cherry-picked to 0.26-stable branch now.

What this change does:
- react-native-cli (major release bump) saves exact versions of react-native and react (only in npm2) dependencies into package.json
- local-cli saves exact version of react (only npm3) dependency into package.json
Closes facebook/react-native#7879

Differential Revision: D3384705

Pulled By: davidaurelio

fbshipit-source-id: d4dff418f9659bd083ae8826433a4e7c0355d03b

generator/index.js

@@ -107,6 +107,6 @@ module.exports = yeoman.generators.NamedBase.extend({
       return;
     }
 
-    this.npmInstall(`react@${reactVersion}`, { '--save': true });
+    this.npmInstall(`react@${reactVersion}`, { '--save': true, '--save-exact': true });
   }
 });