docs(README): add build status badge for CI test-master job

Restores a build status badge to the main README file, pointing to our Jenkins instance. We can always change the target of the badge to another job in the future. Uses the [Embeddable Build Status Plugin](https://wiki.jenkins-ci.org/display/JENKINS/Embeddable+Build+Status+Plugin) for Jenkins.

Live examples:
Test-integration [![Build Status](http://ci.deis.io/buildStatus/icon?job=test-integration)](http://ci.deis.io/job/test-integration/)
Test-master [![Build Status](http://ci.deis.io/buildStatus/icon?job=test-master)](http://ci.deis.io/job/test-master/)
Test-nightly-dart [![Build Status](http://ci.deis.io/buildStatus/icon?job=test-nightly-dart)](http://ci.deis.io/job/test-nightly-dart/)

[skip ci]

Closes #1911.