Remove broken Travis badge (#6364)

The "green" status of this badge was actually last updated 4 years ago,
until I manually ran a master branch test today that made the current
Travis test failures in the React tests visible. But those test failures
seem to be Travis-related and not real (the same tests pass on
CircleCI). Maybe we can completely remove Travis by now, but let's
remove the now-failing badge for now to not scare users.
Signed-off-by: Julius Volz <julius.volz@gmail.com>

README.md

 # Prometheus
 
-[![Build Status](https://travis-ci.org/prometheus/prometheus.svg)][travis]
 [![CircleCI](https://circleci.com/gh/prometheus/prometheus/tree/master.svg?style=shield)][circleci]
 [![Docker Repository on Quay](https://quay.io/repository/prometheus/prometheus/status)][quay]
 [![Docker Pulls](https://img.shields.io/docker/pulls/prom/prometheus.svg?maxAge=604800)][hub]