refactor: datasource return array of releases

lib/datasource/github.js

lib/datasource/npm.js

lib/datasource/packagist.js

lib/datasource/pypi.js

lib/manager/travis/package.js

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

test/datasource/__snapshots__/github.spec.js.snap

test/datasource/__snapshots__/npm.spec.js.snap

test/datasource/__snapshots__/packagist.spec.js.snap

test/datasource/__snapshots__/pypi.spec.js.snap

test/datasource/github.spec.js

test/datasource/npm.spec.js

test/manager/travis/package.spec.js