Update README.md with new GitHub organization

Refs Graylog2/graylog2-server#4086

Author: Jochen Schalanda

README.md

@@ -1,7 +1,7 @@
 NetFlow Plugin for Graylog
 ==========================
 
-[![Build Status](https://travis-ci.org/graylog-labs/graylog-plugin-netflow.svg?branch=master)](https://travis-ci.org/graylog-labs/graylog-plugin-netflow)
+[![Build Status](https://travis-ci.org/Graylog2/graylog-plugin-netflow.svg?branch=master)](https://travis-ci.org/Graylog2/graylog-plugin-netflow)
 
 This plugin provides a NetFlow UDP input to act as a Flow collector that receives data from Flow exporters. Each received Flow will be converted to a Graylog message.