added Travis CI badge

README.md

 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/dee3e6f5cdd240fc80dfdcc1ee419ac8)](https://www.codacy.com/app/coder103/authzforce-ce-core?utm_source=github.com&utm_medium=referral&utm_content=authzforce/core&utm_campaign=Badge_Grade)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/389/badge)](https://bestpractices.coreinfrastructure.org/projects/389)
+[![Build Status](https://travis-ci.org/authzforce/core.svg?branch=develop)](https://travis-ci.org/authzforce/core)
 
 Javadocs: PDP engine [![Javadocs](http://javadoc.io/badge/org.ow2.authzforce/authzforce-ce-core-pdp-engine.svg)](http://javadoc.io/doc/org.ow2.authzforce/authzforce-ce-core-pdp-engine), XACML/JSON extension [![Javadocs](http://javadoc.io/badge/org.ow2.authzforce/authzforce-ce-core-pdp-io-xacml-json.svg)](http://javadoc.io/doc/org.ow2.authzforce/authzforce-ce-core-pdp-io-xacml-json), Test utilities [![Javadocs](http://javadoc.io/badge/org.ow2.authzforce/authzforce-ce-core-pdp-testutils.svg)](http://javadoc.io/doc/org.ow2.authzforce/authzforce-ce-core-pdp-testutils)