Adding the build status icon to the readme file.

README.md

 SimpleSAMLphp
 =============
-
+[![Build Status](https://travis-ci.org/simplesamlphp/simplesamlphp.svg?branch=master)]
+(https://travis-ci.org/simplesamlphp/simplesamlphp)
 This is the official repository of the SimpleSAMLphp software.
 
 * [SimpleSAMLphp homepage](https://simplesamlphp.org)