refactor: Convert Object.assign to spread (#585)

lib/config/index.js

lib/config/migration.js

lib/logger/pretty-stdout.js

lib/workers/branch/index.js

lib/workers/branch/npm.js

lib/workers/branch/yarn.js

lib/workers/package-file/index.js

lib/workers/package/versions.js

lib/workers/pr/index.js

lib/workers/repository/apis.js

lib/workers/repository/index.js

lib/workers/repository/upgrades.js

package.json

test/api/npm.spec.js

test/config/index.spec.js

test/workers/branch/index.spec.js

test/workers/package-file/index.spec.js

test/workers/pr/__snapshots__/index.spec.js.snap

test/workers/pr/index.spec.js

test/workers/repository/cleanup.spec.js