Bump patch version and update CHANGELOG

Author: SammyK

CHANGELOG.md

@@ -6,6 +6,8 @@ Starting with version 5, the Facebook PHP SDK follows [SemVer](http://semver.org
 ## 5.x
 
 Version 5 of the Facebook PHP SDK is a complete refactor of version 4. It comes loaded with lots of new features and a friendlier API.
+- 5.3.1
+  - Fixed a bug where the `polyfills.php` file wasn't being included properly when using the built-in auto loader (#633)
 - 5.3.0
   - Bump Graph API version to v2.7.
 - 5.2.1

src/Facebook/Facebook.php

@@ -53,7 +53,7 @@ class Facebook
     /**
      * @const string Version number of the Facebook PHP SDK.
      */
-    const VERSION = '5.3.0';
+    const VERSION = '5.3.1';
 
     /**
      * @const string Default Graph API version for requests.