Add React.version

getConfig needs to be a function because grunt.config.data.pkg.version isn't available at the time that grunt/config/jsx/jsx.js is required.

Test Plan:
grunt build, grunt lint, grunt test all work. After building, both react.js and react.min.js contain the version number.

src/.jshintrc

@@ -19,6 +19,7 @@
   "unused": "vars",
 
   "globals": {
+    "__VERSION__": false,
     "__DEV__": false,
     "require": false,
     "module": false,

src/core/React.js

@@ -50,4 +50,8 @@ var React = {
   isValidComponent: ReactComponent.isValidComponent
 };
 
+// Version exists only in the open-source version of React, not in Facebook's
+// internal version.
+React.version = __VERSION__;
+
 module.exports = React;