Update CHANGELOG and bump version

Author: SammyK

CHANGELOG.md

@@ -3,9 +3,12 @@
 Starting with version 5, the Facebook PHP SDK follows [SemVer](http://semver.org/).
 
 
-## 5.0.x
+## 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.
+- 5.1.5
+  - Removed mbstring extension dependency
+  - Updated required PHP version syntax in composer.json
 - 5.1.4
   - Breaking changes
     - Changes the serialization method of FacebookApp

README.md

@@ -2,7 +2,7 @@
 
 [![Build Status](https://img.shields.io/travis/facebook/facebook-php-sdk-v4/master.svg)](https://travis-ci.org/facebook/facebook-php-sdk-v4)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/facebook/facebook-php-sdk-v4/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/facebook/facebook-php-sdk-v4/?branch=master)
-[![Latest Stable Version](http://img.shields.io/badge/Latest%20Stable-5.1.4-blue.svg)](https://packagist.org/packages/facebook/php-sdk-v4)
+[![Latest Stable Version](http://img.shields.io/badge/Latest%20Stable-5.1.5-blue.svg)](https://packagist.org/packages/facebook/php-sdk-v4)
 
 
 This repository contains the open source PHP SDK that allows you to access the Facebook Platform from your PHP app.

src/Facebook/Facebook.php

@@ -53,7 +53,7 @@ class Facebook
     /**
      * @const string Version number of the Facebook PHP SDK.
      */
-    const VERSION = '5.1.4';
+    const VERSION = '5.1.5';
 
     /**
      * @const string Default Graph API version for requests.