msys-gzip: Upgrade to 1.6

This version supports deterministic compression/decompression and is therefore a step into the direction of deterministic builds.

Buildbot: https://dscho.cloudapp.net/job/sdk-build-packages/14
This is what the default src_test() also does, so it should go away to simplify the file (see 99c6af064550a44e63c1119173a3a7391fc6058e).