Update third_party/go/pkg/archive/zip to hg rev afc7e22420ce:

Change-Id: I74b92ed2cbee0c963df2adc8f7e85ac69e3162e1
changeset:   20991:afc7e22420ce
user:        Brad Fitzpatrick <bradfitz@golang.org>
date:        Sun Aug 31 21:32:13 2014 -0700
summary:     archive/zip: add Writer.Flush
blobpacked: record meta for blobs packed in zips

Change-Id: I4faff4e22b25bba804562bce318016ed7a6dfd9b