Use Gradle `changing` to determine snapshots

From https://github.com/GoogleContainerTools/jib/pull/584#discussion_r204366857. Gradle supports [explicitly indicating that a dependency may be changing](
https://stackoverflow.com/a/42058780/600339) and [provides an API](https://docs.gradle.org/current/javadoc/org/gradle/api/artifacts/ExternalModuleDependency.html#isChanging--) to check. The [docs](https://docs.gradle.org/current/userguide/declaring_dependencies.html#sub:declaring_dependency_with_changing_version) [hint](https://docs.gradle.org/current/userguide/troubleshooting_dependency_resolution.html#sub:dynamic_versions_and_changing_modules) that SNAPSHOTs are marked as changing.