refactor(versioning): use get function

lib/datasource/github.js

lib/util/package-rules.js

lib/versioning/index.js

lib/workers/pr/changelog/index.js

lib/workers/pr/changelog/source-github.js

lib/workers/pr/pr-body.js

lib/workers/repository/process/lookup/filter.js

lib/workers/repository/process/lookup/index.js

lib/workers/repository/process/lookup/rollback.js

test/versioning/index.spec.js