README.md: Add Travis CI build status (v3.8.x branch)

(cherry picked from commit a0083fd)

Author: dumbbell

README.md

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 # RabbitMQ Server
 
+[![Build Status](https://travis-ci.org/rabbitmq/rabbitmq-server.svg?branch=v3.8.x)](https://travis-ci.org/rabbitmq/rabbitmq-server)
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 [RabbitMQ](https://rabbitmq.com) is a [feature rich](https://rabbitmq.com/documentation.html), multi-protocol messaging broker. It supports:
 
  * AMQP 0-9-1