chore(release): Prepare development of next version

Author: dborsatto

CHANGELOG.md

@@ -3,6 +3,8 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [Unreleased](https://github.com/contentful/contentful-core.php/compare/1.2.0...HEAD)
+
 ## [1.2.0](https://github.com/contentful/contentful-core.php/tree/1.2.0) (2018-08-23)
 
 ### Changed

composer.json

@@ -30,7 +30,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "1.2.0-dev"
+            "dev-master": "1.3.0-dev"
         }
     }
 }