Update minio-go dependency to 2.0.3 to support Amazon S3 Accelerated endpoints

minio-go 2.0.3 has a number of fixes to support cache uploads via Amazon S3 Accelerated transfers, which we'll need for omnibus-gitlab#1804:

https://github.com/minio/minio-go/issues/585

I think we are using 1.0 from: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/v1.9.2/Godeps/Godeps.json#L305