Bump Zlib BB release to `v1.2.11+6`
Fantastic chain of events:
* Zlib BB release `v1.2.11+3` had symlinks in the windows tarball
* These unpacked just fine using Cygwin's `tar` on the windows buildbots
* After building, `7z` sees Cygwin's symlink file contents as a string,
packges it up just fine
* Julia, when installed on a windows machine, unpacks a `libz.dll` that
is a garbage fake symlink file, tries to open `libz.dll` and throws out
a `%1 is not a valid Win32 application.` error.
[0]: https://ci.appveyor.com/project/Ward9250/libz-jl/builds/28054772/job/5jqq9wpk8iaieg6m
