Uses google-cloud-bom to retrieve versions for google-cloud-java artifacts (#258)
Uses google-cloud-bom to retrieve versions for google-cloud-java artifacts (#258)

This way, we're guaranteed to have a set of versions that work well with
each other.

Fixes #257
spring-cloud/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,spring-cloud/spring-cloud-gcp,spring-cloud/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,spring-cloud/spring-cloud-gcp