Show the status for the same branch as the build

Currently the build status image is shown as failed even if the build passes. This happens as image source points to different branch than the build (namely master)

Author: tomasliubinas

README.md

@@ -1,6 +1,6 @@
 # Doctrine2 behavioral extensions
 
-[![Build Status](https://secure.travis-ci.org/Atlantic18/DoctrineExtensions.png?branch=master)](http://travis-ci.org/Atlantic18/DoctrineExtensions)
+[![Build Status](https://secure.travis-ci.org/Atlantic18/DoctrineExtensions.png)](http://travis-ci.org/Atlantic18/DoctrineExtensions)
 [![Latest Stable Version](https://poser.pugx.org/gedmo/doctrine-extensions/version)](https://packagist.org/packages/gedmo/doctrine-extensions)
 
 **Note:** extensions might not evolve more after **2.4.x** it will remain stable and backward compatible. Unless