Fix zlib 1.2.7 not available from server anymore

Only the current version, 1.2.8, is available,
curl http://zlib.net/zlib-1.2.7.tar.gz
returns a 404 Not Found.
Merge pull request #150 from rburgstaler/rb/fix-zlib-not-available-from-server-anymore

Fix zlib 1.2.7 not available from server anymore