refactor: return object from extract (#1914)

lib/manager/bazel/extract.js

lib/manager/buildkite/extract.js

lib/manager/circleci/extract.js

lib/manager/docker-compose/extract.js

lib/manager/docker/extract.js

lib/manager/meteor/extract.js

lib/manager/npm/extract.js

lib/manager/nvm/extract.js

lib/manager/pip_requirements/extract.js

lib/manager/travis/extract.js

lib/workers/package-file/dep-type.js

test/manager/bazel/extract.spec.js

test/manager/buildkite/__snapshots__/extract.spec.js.snap

test/manager/buildkite/extract.spec.js

test/manager/circleci/__snapshots__/extract.spec.js.snap

test/manager/circleci/extract.spec.js

...manager/docker-compose/__snapshots__/extract.spec.js.snap

test/manager/docker-compose/extract.spec.js

test/manager/docker/__snapshots__/extract.spec.js.snap

test/manager/docker/extract.spec.js