Merge pull request #76 from abarth/build_status

Add a build status badge to README.md

README.md

 Sky
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+[![Build Status](https://travis-ci.org/domokit/sky_engine.svg)](https://travis-ci.org/domokit/sky_engine)
+
 Sky is a new way to build high performance, cross platform mobile apps.
 More specifically, Sky is a rendering engine, a scripting engine, an
 (optional) framework, and a (optional) set of Material Design widgets.