Add travis shield

README.md

 # bigdataviewer-core
+[![](https://api.travis-ci.org/bigdataviewer/bigdataviewer-core.svg?branch=master)](https://travis-ci.org/bigdataviewer/bigdataviewer-core)
 [![Join the chat at https://gitter.im/bigdataviewer/bigdataviewer-core](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/bigdataviewer/bigdataviewer-core?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) ImgLib2-based viewer for registered SPIM stacks and more