Make SonarCloud work for pull requests with CircleCI

[plavjanik]

Note from Jack:

I just noticed you are using CircleCI. The environment variable CHANGE_ID here https://github.com/zowe/sample-spring-boot-api-service/blob/master/gradle/sonar.gradle#L3 is a Jenkins variable.

For CircleCI, please change to CIRCLE_PR_NUMBER and see if it works or not. Full built-in environment variables list: https://circleci.com/docs/2.0/env-vars/