Update README.md

Author: vasilakisfil

README.md

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 rubycas-server-core
 ===================
-[![Build Status](https://travis-ci.org/rubycas/rubycas-server-core.png)](https://travis-ci.org/rubycas/rubycas-server-core)
+[![Circle CI](https://circleci.com/gh/vasilakisfil/rubycas-server-core.svg?style=svg)](https://circleci.com/gh/vasilakisfil/rubycas-server-core)
 The core logic for handling CAS requests independent of any particular storage or web presentation technology.