make spring boot applied only when bin directory exists

Problem:

     Spring boot will be applied(detect) when it finds the boot jar in lib/. However during compile time, it was looking for start_script(root), which will fail the push.

E.g. I have a maven built spring boot application. After packaging with war plugin, when I push the war to cf. I am running into this problem. Add one more check condition will fix this.
https://github.com/bijukunjummen/sample-pso-cf-app/

Thanks

-Shaozhen Ding