use gzip readers/writers for artifacts.

The artifacts are very large when not gzip'd and often fail to upload to S3. By gziping them, we minimize the chance that artifacts will fail to upload.

A proper retry logic needs to be implemented for the multipart upload to properly solve this. However, the artifacts should still be gziped.

r: @camilo @fbogsany @shuhaowu 
cc: @Shopify/webscale