z/OS build in CircleCI

[plavjanik]

Resolves #34

• [x] Build z/OS native code on River z/OS system
• [x] Update the SDK commons version in the ZIP file before releasing - resolves #32
• [x] Use Gradle build cache to prevent rebuilding native code when there is no change between builds (both CI and non-CI)

Documentation on how it is done is at https://github.com/zowe/sample-spring-boot-api-service/blob/circleci-zos-build/docs/zos-native-code.md

[plavjanik]

@dkelosky I have added better documentation how to build SDK - https://github.com/zowe/sample-spring-boot-api-service/blob/circleci-zos-build/docs/building.md