feat: rubygems datasource (#3039)

lib/datasource/index.js

lib/datasource/rubygems/errors.js

lib/datasource/rubygems/get.js

lib/datasource/rubygems/index.js

lib/datasource/rubygems/releases.js

lib/datasource/rubygems/retriable.js

test/_fixtures/rubygems/rails/info.json

test/_fixtures/rubygems/rails/versions.json

test/datasource/rubygems/__snapshots__/index.spec.js.snap

test/datasource/rubygems/index.spec.js

test/datasource/rubygems/retriable.spec.js