add link to Travis CI to README

README.md

 # ac
 Access Control: client and server authentication and authorization implementations for user and group management.
 
-- cadcAccessControl - Access control clients and common model objects and exceptions.
-- cadcAccessControl-Server - Access control web service implementation.
-- cadcAccessControl-Admin - Administrative tool for managing users.
-- cadcTomcat - Tomcat 7 authentication realm implementation that uses the AC web service.
+Build status: <a href="https://travis-ci.org/opencadc/ac"><img src="https://travis-ci.org/opencadc/ac.svg?branch=master" /></a>
+ 
+- cadc-access-control - Access control clients and common model objects and exceptions.
+- cadc-access-control-server - Access control web service implementation.
+- cadc-access-control-admin - Administrative tool for managing users.
+- cadc-tomcat - Tomcat 7 authentication realm implementation that uses the AC web service.
 
 Please see the README.md file in each of these modules for build, deployment and customization details.