Change license and remove references to PATENTS

Only remaining references:

```
docs/_posts/2014-10-28-react-v0.12.md
51:You can read the full text of the [LICENSE](https://github.com/facebook/react/blob/master/LICENSE) and [`PATENTS`](https://github.com/facebook/react/blob/master/PATENTS) files on GitHub.

docs/_posts/2015-04-17-react-native-v0.4.md
20:* **Patent Grant**: Many of you had concerns and questions around the PATENTS file. We pushed [a new version of the grant](https://code.facebook.com/posts/1639473982937255/updating-our-open-source-patent-grant/).

src/__mocks__/vendor/third_party/WebComponents.js
8: * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
```

contributing/how-to-contribute.md

@@ -156,7 +156,7 @@ React team meets once a week to discuss the development of React, future plans,
 
 ### License
 
-By contributing to React, you agree that your contributions will be licensed under its BSD license.
+By contributing to React, you agree that your contributions will be licensed under its MIT license.
 
 ### What Next?