README: point travis build status badge to v3.7.x

As we don't support v3.8.x (master) yet, the build will fail.

Signed-off-by: Matteo Cafasso <[email protected]>

Author: noxdafox

README.md

@@ -1,6 +1,6 @@
 # RabbitMQ Message Deduplication Plugin
 
-[![Build Status](https://travis-ci.org/noxdafox/rabbitmq-message-deduplication.svg?branch=master)](https://travis-ci.org/noxdafox/rabbitmq-message-deduplication)
+[![Build Status](https://travis-ci.org/noxdafox/rabbitmq-message-deduplication.svg?branch=v3.7.x)](https://travis-ci.org/noxdafox/rabbitmq-message-deduplication)
 
 A plugin for filtering duplicate messages.