README: add travis button

[ci skip]

README.md

@@ -1,6 +1,7 @@
 Qubes core, version 3
 ---------------------
 
+[![Build Status](https://travis-ci.org/woju/qubes-core-admin.svg?branch=core3-devel)](https://travis-ci.org/woju/qubes-core-admin)
 <img src="https://readthedocs.org/projects/qubes-core-admin/badge/?version=latest">
 
 This is development branch of the Qubes OS core. This branch is subject to