Fix TravisCI build status link following rename

Correct the TravisCI link used for the build status image following the
GitHub organization rename from SpringSource to spring-projects.

README.md

-# Spring Boot [![Build Status](https://travis-ci.org/SpringSource/spring-boot.png?branch=master)](https://travis-ci.org/SpringSource/spring-boot)
+# Spring Boot [![Build Status](https://travis-ci.org/spring-projects/spring-boot.png?branch=master)](https://travis-ci.org/spring-projects/spring-boot)
 Spring Boot makes it easy to create Spring-powered, production-grade applications and
 services with absolute minimum fuss. It takes an opinionated view of the Spring platform
 so that new and existing users can quickly get to the bits they need.