Update Travis CI badge in README

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Author: Jochen Schalanda

README.md

@@ -1,11 +1,11 @@
 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/graylog-labs/graylog-plugin-netflow.svg?branch=2.2)](https://travis-ci.org/graylog-labs/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.
 
-**Required Graylog version:** 2.3.0 and later
+**Required Graylog version:** 2.2.0 and later
 
 ## Supported NetFlow Versions