Auto merge of #3719 - alexcrichton:curl-sys, r=alexcrichton
Auto merge of #3719 - alexcrichton:curl-sys, r=alexcrichton

Update libz-sys

Try to fix AppVeyor CI

rust-lang/cargo,tsurai/cargo,durka/cargo,vojtechkral/cargo,vojtechkral/cargo,tshepang/cargo,achanda/cargo,rustyhorde/cargo,tshepang/cargo,durka/cargo,rust-lang/cargo,rust-lang/cargo,durka/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,achanda/cargo,nipunn1313/cargo,vojtechkral/cargo,tsurai/cargo,tshepang/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,eminence/cargo,durka/cargo,rust-lang/cargo,tshepang/cargo,achanda/cargo,rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,achanda/cargo