Update gzip_cache to produce consistent .gz files.

The gzip protocol includes the timestamp in the file
by default. This causes the contents of the file to
be slightly different every time, causing problems
for some long-term caching strategies.

This patch removes the timestamp, allowing gzip to produce
the same file from the same source every time.
Merge pull request #226 from pmclanahan/deterministic-gzip

Update gzip_cache to produce consistent .gz files.