Fix travis badge using old project name

Author: rtheunissen

README.md

@@ -1,6 +1,6 @@
 # PHP Decimal
 
-[![Build Status](https://travis-ci.org/php-decimal/extension.svg?branch=master)](https://travis-ci.org/php-decimal/extension)
+[![Build Status](https://travis-ci.org/php-decimal/ext-decimal.svg?branch=master)](https://travis-ci.org/php-decimal/extension)
 [![Build status](https://ci.appveyor.com/api/projects/status/lg5nw5tqgpmv1c33?svg=true)](https://ci.appveyor.com/project/rtheunissen/php-decimal)
 [![PECL](https://img.shields.io/badge/PECL-1.0.1-blue.svg)](https://pecl.php.net/package/decimal)