README.md: embed the Travis CI badge

It should make it easier to keep track of the status
of daily cron jobs uploading data to Coverity Scan.

README.md

@@ -4,7 +4,8 @@
 <a href="https://in.waw.pl/systemd-github-state/systemd-systemd-pull-requests.svg"><img align="right" src="https://in.waw.pl/systemd-github-state/systemd-systemd-pull-requests-small.svg" alt="Count of open pull requests over time"></a>
 [![Build Status](https://semaphoreci.com/api/v1/projects/28a5a3ca-3c56-4078-8b5e-7ed6ef912e14/443470/shields_badge.svg)](https://semaphoreci.com/systemd/systemd)<br/>
 [![Coverity Scan Status](https://scan.coverity.com/projects/350/badge.svg)](https://scan.coverity.com/projects/350)<br/>
-[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1369/badge)](https://bestpractices.coreinfrastructure.org/projects/1369)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1369/badge)](https://bestpractices.coreinfrastructure.org/projects/1369)<br/>
+[![Build Status](https://travis-ci.org/systemd/systemd.svg?branch=master)](https://travis-ci.org/systemd/systemd)
 
 ## Details